However, if the placenta is less than 2cm from the opening of the uterus into the cervix, which is called the internal os at the 18-20 week scan, it may still be low at the time of the birth. Therefore, many obstetricians and midwives ask for it to be re-checked in the third trimester.

Please be assured, there is no need to worry unduly about this finding as about 10-15% of placentas are low-lying at the 18-20 week scan, but only 0.5% are still low-lying by full term.

How do I arrange for a placental location scan?

Ultrasound Care’s experienced team of specialist obstetricians and sonographers perform placental location ultrasounds in 8 different locations. To arrange for your placental location scan, please call an Ultrasound Care practice that is the most convenient for you, we have locations all around Sydney.

Sonography 3rd Trimester And Placenta Assessment, Protocols, And Interpretation - StatPearls

Continuing Education Activity

During the third trimester of pregnancy, ultrasound is commonly performed in patients that present both asymptomatically or with symptoms. There are currently no major guidelines or protocols to standardize the use of ultrasound at this stage of pregnancy. Ultrasound can be useful to identify fetal and maternal pathology and to follow the progression of the pregnancy. This activity reviews the approach and interpretation of ultrasound during the third trimester and highlights relevant information for an interprofessional team managing this patient population.

Objectives:

• Describe the general technique to perform an ultrasound evaluation during the third trimester of pregnancy.

• Identify the indications and contraindications for performing an ultrasound evaluation during the third trimester of pregnancy.

• Describe the typical fetal and maternal ultrasonographic findings during the third trimester of pregnancy.

• Summarize the most common pathologic findings that can be appreciated when performing an ultrasound evaluation during the third trimester of pregnancy.

Access free multiple choice questions on this topic.

Introduction

The use of ultrasound in the third trimester of pregnancy serves a multitude of general and specialized purposes that include but are not limited to the determination of fetal number and presentation, assessment of growth disorders, and characterization of the placenta and amniotic fluid. Thus, the ultrasonographic applications in the third trimester of pregnancy differ from previous trimesters in both scope and objectives.

Traditionally, the third trimester is defined as the antenatal period between 28 and 42 weeks of gestation. There is currently no universal, standardized protocol for assessing the progression and status of the pregnancy during this period. 

Sonographic assessments in the third trimester are not routinely performed unless the patient has had no initial sonographic examination or prior pathological or concerning conditions are identified during the first or second trimesters.[1]

The purpose of the study serves as a guide for what is evaluated during the examination. Further detailed studies about abnormal findings are described more extensively under their respective activities.

Anatomy and Physiology

During the third trimester, ultrasound can be performed transvaginally or trans-abdominally, with preference given to the latter approach due to the theoretical risk of precipitating clinically significant hemorrhage if placenta preview is present and undiagnosed.[2] The transvaginal approach generally offers better visualization of the anatomy due to the improved proximity to the organs and structures of interest. However, due to the higher frequency transducer used, this can result in difficulty visualizing more caudal (maternal) and deeper structures inside and outside the uterus.  

The uterus is typically found wedged between the more anterior bladder and the more posterior colon. In a cephalad to caudal direction, the fundus, body, and cervix of the uterus can be appreciated in that particular order. During the third trimester, the fetal anatomy and cardiac activity should be well defined and evident.

The amniotic fluid should be seen as an anechoic space surrounded by the isoechoic walls of the uterus. The location of the placenta is variable, and it can be identified as a heterogeneous structure connected to the wall of the uterus and attached to the umbilical cord leading to the abdomen of the fetus.[3]

Indications

In an asymptomatic, gravid female, third-trimester ultrasound is indicated for the evaluation and determination of:[1]

1. Fetal anatomy

2. Fetal anomalies

3. Gestational age

4. Fetal growth

5. Fetal presentation

6. Suspected multiple gestations

7. Placental location

8. Cervical insufficiency

 Third-trimester ultrasound examination on the symptomatic pregnant patient has multiple indications, including, but not limited to, the following conditions:[1]

1. The discrepancy between the uterine size and calculated gestational date

2. Pelvic mass

3. Ectopic pregnancy

4. Fetal death

5. Fetal anomaly

6. Vaginal bleeding

7. Abdominal or pelvic pain

8. Hydatidiform mole

9. Uterine abnormalities

10. Fetal abnormalities

11. Amniotic fluid abnormalities

12. Placental abruption

13. Premature rupture of membranes

14. Premature labor

15. Placenta previa

Additionally, other applications of ultrasonography include acting as an adjunct to procedures, e. g., amniocentesis, cervical cerclage placement, external cephalic version. 

A particular patient population that is especially benefited from using ultrasound in the third trimester is the late presenter without prior prenatal care. Unfortunately, these patients are prone to all aforementioned disorders, and an ultrasonographic evaluation is of paramount importance to avoid, or at least plan for, significant morbidity and mortality causes. 

A more detailed and specialized examination of fetal anatomy and characteristics of the placenta may be required under certain clinical circumstances.[4] These protocols and applications are beyond the scope of the objectives of this chapter.

Contraindications

There are few contraindications for the use of transabdominal ultrasound during the third trimester of pregnancy. The only universally accepted absolute contraindication is patient refusal, following the principle of patient autonomy.  

Transvaginal ultrasound is relatively contraindicated during the third trimester, especially in patients without a prior documented placental location, due to concerns for precipitating bleeding in the setting of undiagnosed placenta previa. It is recommended to begin with a transabdominal ultrasound to locate the placenta first, followed by the transvaginal ultrasound if clinically indicated and necessary. However, due to the cervix and the vaginal canal angle, performing transvaginal ultrasound during the third trimester can be potentially safe. The risk of precipitating clinically significant hemorrhage has not been consistently supported in the literature.[5]

Equipment

For appropriate sonographic evaluation, two probe transducers must be available.  A curvilinear probe (1 to 6 MHz) is the preferred probe for transabdominal assessment, while a high-frequency endocavitary probe (7.5 to 10 MHz) is used for transvaginal evaluation.

A probe cover must be used for the transvaginal approach. After each use, the probe should be appropriately disinfected as per institution protocols.

Preparation

If performing a transabdominal study, the patient should be asked to have a full bladder if possible as fluid-filled structures enhance the resolution of deeper structures on ultrasound as it aids in examining the uterus, placenta, and amniotic fluid.  

If a transvaginal ultrasound is deemed necessary, the patient should be asked to empty their bladder. Given the proximity of the intracavitary probe to the cervix, an empty bladder provides increased visualization of structures and improves patient comfort during the study.[6]

Technique

The initial evaluation should commence by placing a curvilinear probe with the appropriately selected OB settings over the suprapubic area. The probe can be placed in either sagittal or transverse orientation. The probe indicator should be directed to the cephalad for the sagittal images and towards the patient's right side for transverse images. 

If a transvaginal examination is indicated, the patient should be placed in the lithotomy position. The probe should be inserted with the indicator in the vertical direction. Sagittal images are obtained by maintaining the indicator pointing towards the ceiling, while transverse imaging requires rotating the probe to have the indicator facing the patient's right side. [4]

Several measurements are indicated for appropriate assessment of the status of the fetus and the maternal structures, starting with the estimation of the gestational age. Gestation dating by ultrasonography may be inaccurate during the third trimester.[7] Measurements of fetal parts can be used in isolation or combined to estimate gestational age. Commonly used dating measurements include: 

• Biparietal diameter (BPD)

• Occipitofrontal diameter (OFD)

• Head circumference (HC)

• Abdominal diameter (AD)

• Abdominal circumference (AC)

• Femur length (FL)

On the other hand, estimation of fetal size is best performed during the third trimester.[8] Multiple formulas have been developed to estimate fetal weight, with one of the most widely used being that of Hadlock and colleagues. Formulas that used less than three fetal body part measurements did not perform well compared to formulas that include fetal head, abdomen, and femur measurements. [9] However, including additional measurements in the formulas has not demonstrated increased accuracy.[10]

Frequently, fetal movement can be identified and should be documented. However, lack of fetal movement can have different significances and can be affected by the fetal sleep cycle. The fetal heart should be measured using M mode as established by As Low As Reasonably Achievable (ALARA) recommendations.[11] The M mode caliper should be placed over the left ventricle. By measuring the temporal distance in M mode from peak to peak, most modern ultrasound machines will be able to estimate the fetal heart rate. Care must be applied to the correct number of cycles measured since some manufacturers will require two cycles to be measured, and some only one. A normal heart rate is between 110 and 160 beats per minute. Further discussion about abnormal findings is discussed below. 

Evaluating the fetal lie is of paramount importance, especially as the term date approaches. First, the fetal head is identified, followed by the direction of the hyperechoic fetal spine. If the fetal head is closer to the cervix, this is described as cephalad. The laterality of the spine is also essential, and it is always described based on the mother's orientation. On the other hand, when the head is more cephalad to the mother, it is considered a breech presentation. The specific type of breech presentation should be evaluated as frank, complete, or footling.[12]

The fetal anatomy should be evaluated for any apparent abnormalities. The laterality of all anatomic structures visualized should be noted. The fetal head and its general should be examined for symmetry. Intracranial anatomical structures should be visible, including the cerebellum, cavum septum pellucidum, and ventricles. The inability to identify any of these structures warrants further investigation.[13]

The probe should be optimized to assess the heart's movement, position, and orientation in detail. During the third trimester, the four heart chambers should be visible. The fetal heart rate should be evaluated for any signs of arrhythmia. The lungs should be seen flanking the heart bilaterally, and they should appear echogenic and homogeneous, in what is described as "liver-like."[14]

Further moving caudally in the fetus, the abdominal organs should be appreciated. The stomach is seen on the left side as an anechoic cystic structure below the diaphragm.[15] Bilaterally, the kidneys should present symmetrically, and the bladder should be seen in the inferior abdomen as a small hypoechoic cystic structure.[16] From the anterior abdominal wall, the insertion of the umbilical cord should be evaluated for any sign of extrusion of intrabdominal organs. Finally, the long bones and the spine should be followed and assessed for symmetry. The spine should be symmetric, hyperechoic, and should taper to an intact posterior skin edge.[17] The position, movement, and tone of the extremities should also be examined. 

A general evaluation of the cervix is wise. First, by visual inspection, the sonographer should determine if the cervix is open or closed. If open, the distance from the inner wall to the inner wall should be measured. Subsequently, due to the morbidity associated with cervical insufficiency, the length from the outer to the inner cervical os should also be measured.[18]

The placenta can be identified by tracing the endometrium until an isoechoic structure with increased vascularity on color Doppler. The location of the placenta should be described as anterior or posterior. The location of the placenta should be described as anterior or posterior. The presence of placenta previa should be excluded by measuring the distance from the caudal edge of the placenta to the inner cervical os. A measurement of less than 3 cm is concerning for placenta previa and should be further evaluated.[19]

The placenta should be further inspected for masses or retroplacental hemorrhage, which will appear as heterogenous or anechoic areas, respectively. Its size in both longitudinal and transverse axes should be determined. Any abnormal finding should be further studied by applying both Color Doppler and Power Doppler modes.  

From the placenta, the point of insertion of the umbilical cord should be traced and classified as central, eccentric, marginal, or velamentous. After applying color Doppler mode to the umbilical cord, both venous and arterial flow pulsations should be distinguished. Ideally, spectral doppler should be seen in the free cord, but this can be technically challenging at times. In these cases, consistency should be preferred over accuracy, and it should be measured at a site where it can be reliably reproduced. Once a continuous wave Doppler is applied, an umbilical artery waveform following a “sawtooth” pattern should be easily discerned from the venous flow. A resistive index can be calculated by applying the formula as follows:

resistive index (RI) (Pourcelot index) = (peak systolic velocity – end-diastolic velocity) / peak systolic velocity

Doppler indices are known to decline with the progression of the gestational age, given that diastolic flow increases gradually. If the umbilical artery resistive index is abnormal, further investigation is warranted.[20]

Finally, the amniotic fluid level should be measured by both amniotic fluid index (AFI) and deepest vertical pocket (DVP) methods if possible. To assess AFI, using the linea nigra and mediolateral lines as the axial and horizontal axis, the sonographer must divide the uterus into four different quadrants.

The deepest pocket identified in each quadrant devoid of both fetal parts and the umbilical cord is measured vertically. The dimensions in centimeters of these measurements are added together to calculate the AFI. Normal AFI is between 5 to 25 cm. An AFI of less than 5 cm is considered oligohydramnios.[21]

To determine DVP, the deepest pocket of fluid devoid of fetal parts and the umbilical cord is identified and measured vertically. A value of less than 2 cm is concerning for oligohydramnios, while more than 8 cm indicates polyhydramnios.[22]

As stated before, abnormal findings required further specialized sonographic examination and prompt consultation with an obstetrician. Other specialized studies fall beyond the scope of this activity.

Complications

The most significant risk of complication can result from a transvaginal ultrasound approach that can disrupt the integrity of a placenta previa, leading to iatrogenic hemorrhage. Due to the angle of orientation between the placement of the endocavitary probe and the position of the cervix, some studies have argued that this risk is potentially overstated. However, there are no major risks associated with the alternative of using a transabdominal approach to the evaluation of the gestation during the third trimester, so this should be preferred and attempted first.

Other minor risks associated with the use of ultrasound entail the anxiety that it can elicit on the patient and the discomfort associated with the pressure of the probe against the skin or vaginal mucosa.[23]

Clinical Significance

The initial component of the ultrasonographic evaluation should be the determination of the number of fetuses. There is an increased risk of morbidity and mortality associated with unexpected multiple gestations. Common errors that can lead to misidentification of the correct plurality include not performing a full scan, including the uterus's fundus, or not tracking the fetal head continuously to its corresponding body.[24]

Subsequently, once a fetus is identified, viability should be assessed by evaluating cardiac motion and fetal heart. Fetal heart rate also provides additional prognostic value.  A slow fetal heart rate has been associated with a poorer prognosis, but this should not be the sole determination to consider the pregnancy nonviable.[25]

Fetal lie and presentation are of higher importance in the third trimester than the first and second trimester. Change in these two parameters decreased as the due date approaches. Fetal lie refers to the angle of alignment of the fetus and uterus in the sagittal axis. Longitudinal lie, the most commonly found, refers to both the fetal and uterine axis aligning in parallel.  The other two possibilities include a transverse, and less commonly, an oblique lie.[12]

Presentation, on the other hand, refers to the fetal anatomic part closer to the cervix. The cephalic position is the most commonly found with the fetal head as the presenting part, and it correlates with improved delivery outcomes. Malpresentation, including breech, footling, or kneeling, is often associated with increased labor difficulty and, consequently, with increased morbidity and mortality. Ultrasound can determine both complete and incomplete variations of presentation. Both fetal lie and presentation must be assessed concurrently since an incomplete examination can misinterpret the findings. If the fetal head is closer to the cervix or lower segment of the uterus, the fetal lie in the sagittal axis should be confirmed before reporting a cephalic presentation since the fetus could be in a transverse axis. If a malpresentation is identified, the ultrasonographer should assess and evaluate potential causes, including placental abnormalities or fetal malformations. [26]

Estimating gestational age and weight should be attempted with the caveat that measurements in the third trimester are less accurate than those made during the first trimester. Fetal abnormalities can also affect the measurements of different body parts. 

An important aspect of the examination in the third trimester is the evaluation of the amniotic fluid. There remains some controversy about the best methodology when there are moderate abnormalities in fluid volume, but more concordance exists at the extremes. The initial evaluation should subjectively allow for assessment for polyhydramnios and oligohydramnios. It is important to remember that near-term, amniotic fluid volume may appear decreased due to the relatively larger size of the fetus. Objective measurements should follow either using the deepest vertical pocket method (DVP) or amniotic fluid index (AFI). A diagnosis of oligohydramnios should prompt emergent referral and assessment by an obstetrician due to the associated prognosis of fetal demise. On the other hand, polyhydramnios requires a less emergent consultation but should prompt further evaluation and testing since it can be associated with complications for the mother and the fetus. In the case of multiple gestations, an increased amniotic fluid volume must raise immediate suspicion for an etiology related to a fetal abnormality. The majority of cases are related to twin to twin transfusion syndrome.[27]

Determining the location of the placenta and its relation to the cervix is a critical aspect of the examination. If performing a transabdominal ultrasound examination, various maneuvers can correctly assess the placenta, including changing patient position. If this fails, a transvaginal exam can be considered. If the diagnosis of placenta previa cannot be excluded despite all efforts, one must be cautious and assume the presence of the diagnosis. In patients with prior cesarean section in which a placenta previa is identified, further detail should be directed towards assessing for placenta accreta. [28]

In patients that present for abdominal pain, a significant concern is a placental abruption. However, ultrasound is not ideal for identifying placental abruption as both false positives, and false negatives are common. The myometrium and its supplying vasculature can frequently be mistaken for a hematoma. On the other hand, a small abruption can be easily confused for normal myometrium.[29]

Ultrasonography also offers the opportunity to evaluate the umbilical cord and determine the presence of vasa previa. Findings that should raise suspicion for this diagnosis include resolved placenta previa, succenturiate placental lobe, and velamentous cord insertion.[28]

A commonly asked question during the ultrasonographic examination is the presence of fetal abnormalities. It is impractical to examine every patient for every possible abnormality. However, major abnormalities are usually evident on a routine examination. Determining what is considered reassuring in the absence of any obvious abnormality remains debated, given the complexity of the issue. Furthermore, abnormalities detected during the first and second trimester of pregnancy can disappear in the third trimester. Opposite to that, some abnormalities only become apparent with the advancement of gestation. It is currently recommended that if only one ultrasound is performed, that it should be done at 18 to 20 weeks of gestation. Any malformation found should be further evaluated by a sonographer with expertise in the precise abnormality discovered.

Enhancing Healthcare Team Outcomes

The approach to taking care of the pregnant patient during the third trimester of pregnancy requires an interdisciplinary effort involving physicians, nurses, midwives, pharmacists, and other healthcare team members. There are multiple factors to consider during this period, and ultrasound has evolved as an essential tool to evaluate the progression of pregnancy. Even though there is no universal recommendation to obtain a routine ultrasound in the third trimester, its use can help identify pathology that can directly affect the outcome of both the patient and the pregnancy. Understanding the applications and limitations of this tool can help reduce maternal and neonatal morbidity and mortality.

Nursing, Allied Health, and Interprofessional Team Interventions

Ultrasound examination at the bedside and performed by an experienced provider should be considered in all pregnant patients present during the third trimester. Patients who have not received prenatal care benefit greatly from its use as undiagnosed pathology can be identified. The function of ultrasound during this period is to identify pathology that could have been missed as well as to determine factors that directly affect the delivery, e.g., fetal lie, fetal size, fetal presentation, placental location. The ultrasound examination must be thorough and yet focused. Its completion includes an examination of the fetus, placenta, amniotic fluid, umbilical cord, and cervix.

Nursing, Allied Health, and Interprofessional Team Monitoring

An important advantage of the ultrasound examination is its reproducibility. Repeat examinations can be performed if dynamic changes are suspected. Additionally, patients with identified pathology should receive a specialized examination that is beyond the scope of this article. Nursing should be prepared and familiar with the terminology of ultrasound findings to monitor and provide assistance in the delivery of patients with pathology.

Review Questions

• Access free multiple choice questions on this topic.

• Comment on this article.

Figure

Transvaginal ultrasound demonstrating placenta previa. Contributed and Used with permission from New York Methodist Hospital

Figure

Third trimester pregnancy ultrasound with oligohydramnios. Contributed and Used with permission from New York Methodist Hospital

References

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Pooh RK. Normal anatomy by three-dimensional ultrasound in the second and third trimesters. Semin Fetal Neonatal Med. 2012 Oct;17(5):269-77. [PubMed: 22817865]

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Timor-Tritsch IE, Yunis RA. Confirming the safety of transvaginal sonography in patients suspected of placenta previa. Obstet Gynecol. 1993 May;81(5 ( Pt 1)):742-4. [PubMed: 8469465]

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Laing FC. Placenta previa: avoiding false-negative diagnoses. J Clin Ultrasound. 1981 Mar;9(3):109-13. [PubMed: 6783681]

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Verburg BO, Steegers EA, De Ridder M, Snijders RJ, Smith E, Hofman A, Moll HA, Jaddoe VW, Witteman JC. New charts for ultrasound dating of pregnancy and assessment of fetal growth: longitudinal data from a population-based cohort study. Ultrasound Obstet Gynecol. 2008 Apr;31(4):388-96. [PubMed: 18348183]

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Milner J, Arezina J. The accuracy of ultrasound estimation of fetal weight in comparison to birth weight: A systematic review. Ultrasound. 2018 Feb;26(1):32-41. [PMC free article: PMC5810856] [PubMed: 29456580]

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Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Estimation of fetal weight with the use of head, body, and femur measurements--a prospective study. Am J Obstet Gynecol. 1985 Feb 01;151(3):333-7. [PubMed: 3881966]

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Hadlock FP, Harrist RB, Carpenter RJ, Deter RL, Park SK. Sonographic estimation of fetal weight. The value of femur length in addition to head and abdomen measurements. Radiology. 1984 Feb;150(2):535-40. [PubMed: 6691115]

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Committee Opinion No. 723: Guidelines for Diagnostic Imaging During Pregnancy and Lactation. Obstet Gynecol. 2017 Oct;130(4):e210-e216. [PubMed: 28937575]

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Ghosh MK. Breech presentation: evolution of management. J Reprod Med. 2005 Feb;50(2):108-16. [PubMed: 15755047]

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Falco P, Gabrielli S, Visentin A, Perolo A, Pilu G, Bovicelli L. Transabdominal sonography of the cavum septum pellucidum in normal fetuses in the second and third trimesters of pregnancy. Ultrasound Obstet Gynecol. 2000 Nov;16(6):549-53. [PubMed: 11169349]

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Cooper C, Mahony BS, Bowie JD, Albright TO, Callen PW. Ultrasound evaluation of the normal fetal upper airway and esophagus. J Ultrasound Med. 1985 Jul;4(7):343-6. [PubMed: 3892034]

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Nyberg DA, Mack LA, Patten RM, Cyr DR. Fetal bowel. Normal sonographic findings. J Ultrasound Med. 1987 Jan;6(1):3-6. [PubMed: 3546721]

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Arger PH, Coleman BG, Mintz MC, Snyder HP, Camardese T, Arenson RL, Gabbe SG, Aquino L. Routine fetal genitourinary tract screening. Radiology. 1985 Aug;156(2):485-9. [PubMed: 3892578]

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Filly RA, Simpson GF, Linkowski G. Fetal spine morphology and maturation during the second trimester. Sonographic evaluation. J Ultrasound Med. 1987 Nov;6(11):631-6. [PubMed: 3316699]

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Berghella V, Bega G, Tolosa JE, Berghella M. Ultrasound assessment of the cervix. Clin Obstet Gynecol. 2003 Dec;46(4):947-62. [PubMed: 14595237]

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Silver RM. Abnormal Placentation: Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol. 2015 Sep;126(3):654-668. [PubMed: 26244528]

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Comas M, Crispi F, Gómez O, Puerto B, Figueras F, Gratacós E. Gestational age- and estimated fetal weight-adjusted reference ranges for myocardial tissue Doppler indices at 24-41 weeks' gestation. Ultrasound Obstet Gynecol. 2011 Jan;37(1):57-64. [PubMed: 21046540]

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Jaba B, Mohiuddin AS, Dey SN, Khan NA, Talukder SI. Ultrasonographic determination of amniotic fluid volume in normal pregnancy. Mymensingh Med J. 2005 Jul;14(2):121-4. [PubMed: 16056194]

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Magann EF, Sanderson M, Martin JN, Chauhan S. The amniotic fluid index, single deepest pocket, and two-diameter pocket in normal human pregnancy. Am J Obstet Gynecol. 2000 Jun;182(6):1581-8. [PubMed: 10871481]

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Schiffer V, van Haren A, De Cubber L, Bons J, Coumans A, van Kuijk SM, Spaanderman M, Al-Nasiry S. Ultrasound evaluation of the placenta in healthy and placental syndrome pregnancies: A systematic review. Eur J Obstet Gynecol Reprod Biol. 2021 Jul;262:45-56. [PubMed: 33984727]

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Morin L, Lim K. No. 260-Ultrasound in Twin Pregnancies. J Obstet Gynaecol Can. 2017 Oct;39(10):e398-e411. [PubMed: 28935062]

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Yuan SM. Fetal arrhythmias: Surveillance and management. Hellenic J Cardiol. 2019 Mar-Apr;60(2):72-81. [PubMed: 30576831]

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Neilson DR. Management of the large breech infant. A survey of 203 cases from Emanuel Hospital. Am J Obstet Gynecol. 1970 Jun 01;107(3):345-8. [PubMed: 5445004]

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Stagnati V, Zanardini C, Fichera A, Pagani G, Quintero RA, Bellocco R, Prefumo F. Early prediction of twin-to-twin transfusion syndrome: systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2017 May;49(5):573-582. [PubMed: 27270878]

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O'Brien JM. Placenta previa, placenta accreta, and vasa previa. Obstet Gynecol. 2007 Jan;109(1):203-4; author reply 204. [PubMed: 17197611]

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Oyelese Y, Ananth CV. Placental abruption. Obstet Gynecol. 2006 Oct;108(4):1005-16. [PubMed: 17012465]

Ultrasound of the fetoplacental complex

A healthy lifestyle during pregnancy is a matter of course. However, it is sometimes very difficult for some expectant mothers to abandon the usual rhythm of life, especially if the pregnancy was not planned, and the news of it becomes an unexpected surprise. Why is it impossible and necessary to comply with a number of diseases, preventing diseases, giving up bad habits so important during pregnancy? There are at least two answers to the question asked. First, your body needs such protection so that pregnancy does not lead to overload and the development of diseases. And the second is to reduce the influence of harmful factors on the baby, because if a pregnant woman does not adhere to a healthy lifestyle, the permeability of the placenta increases and the risk of damage to the growing body by harmful factors increases. nine0003

What is the placenta for?

During fetal development, the placenta takes over the functions of most organs of the fetus - it nourishes, protects, and provides respiration. Its value as a whole is extremely difficult to overestimate. In addition to ensuring the life of the baby, the placenta is responsible for the normal course of pregnancy. It synthesizes hormones that are necessary to maintain pregnancy.

Damaging factors

Unfortunately, the possibilities of the placental barrier are quite limited, especially when it comes to severe damaging factors such as alcohol, smoking, drugs, and medications. The toxic effect of alcohol on body cells has been studied quite well. At what we are talking now about an adult mature organism, whose cells are not able to withstand the decay products of ethyl alcohol. Therefore, it is not worth talking about its harm to developing, maturing cells - it can be so colossal that it leads to the death of the fetus. A single use of alcohol during pregnancy can lead to its interruption, to disruption of the formation and maturation of nerve cells. Only one tablespoon of the mother's body can neutralize, until the alcohol reaches the placenta, the rest of the volume will safely go to the baby. Nicotine is able not only to increase the permeability of the placenta for harmful factors, but also leads to irreparable consequences - the death of the baby at any stage of pregnancy. May cause miscarriage or premature birth. At the mention of drugs, the figures generally cause a state of shock - more than 80% of pregnancies end in fetal death in utero, and only 20% of pregnancies end in the birth of live children, but their adaptive abilities are significantly reduced, and some of them die in early infancy. Drugs lead to intrauterine death of the fetus, to the development of various defects. nine0003

Fetoplacental insufficiency

Under the influence of stress and chronic diseases of the mother and the above factors, changes occur in the fetoplacental system that lead to disruption of its functions. In the placenta, the number of vessels decreases, their qualitative changes occur, calcium is deposited in the vascular spaces, and cysts form. Gradually, the processes worsen, chronic placental insufficiency develops, which disrupts the normal development of the baby. The child receives less oxygen and nutrients, its development slows down, weight gain stops, the fetus may even die if help is not provided in time. Thanks to modern methods of diagnosis and treatment, the approach to HFPI has changed significantly in recent years. Previously, the only method that helped save the baby's life was considered early delivery by caesarean section. Now it is possible to prolong pregnancy during treatment in order to enable the baby to leave mother's tummy as late as possible. nine0003

Diagnostics

It has already been said above that the occurrence of FPI can occur for various reasons. That is why the diagnosis of placental insufficiency should be a comprehensive examination of a pregnant woman.

The doctor, collecting an anamnesis, finds out the main factor that contributed to the emergence of this problem (age, living and professional conditions, bad habits, the presence of extragenital and gynecological diseases, etc.). A woman in a position with FPI may complain of abdominal pain, the presence of bloody discharge from the vagina, excessive fetal activity or lack of movement, increased uterine tone. nine0003

The gynecologist, conducting a physical examination before determining placental insufficiency, measures the circumference of the abdomen of the expectant mother, evaluates the standing of the uterine fundus, determines the weight of the woman. Thanks to the data obtained, you can find out whether the fetus is developing normally or there is a developmental delay.

Based on the results of a gynecological examination, it is possible to assess the nature of the discharge, detect inflammation, and take material for microscopic and bacteriological examination. nine0003

Ultrasound is of great importance in the detection of fetoplacental insufficiency. Thanks to it, it is possible to determine fetometric indicators (sizes of the head, limbs, torso of the fetus) and compare them with normal values ​​characteristic of a given gestational age, measure the thickness of the placenta and determine its degree of maturity.

If FPI is suspected, the doctor will perform cardiotocography and phonocardiography to evaluate the child's heart activity. Arrhythmia, bradycardia, tachycardia may be signs of hypoxia. nine0003

Dopplerography of the uterine blood flow allows you to evaluate the blood circulation in the vessels of the uterus, umbilical cord, fetal part of the placenta.

Imaging of placenta previa | Blog RH

By Greg Marrinan

Placenta previa (PP) occurs in 0. 3-2.0% of all pregnancies. As the age of the mother increases, the risk of this condition increases.

Placenta previa is a condition in which placental tissue is abnormally close to the inside of the cervix. There are 4 generally recognized subtypes of PP:

1. Complete or central, in which the placenta completely covers the internal os of the uterus;
2. Incomplete or partial, in which the placenta partially covers the internal os of the uterus;
3. Marginal, in which placental tissue adheres to, but does not cover, the internal os of the uterus;
4. A low lying placenta that is abnormally close to the internal uterine os but does not overlap it at all.

The reported incidence of placenta previa in the second trimester is almost 10 times higher than at delivery. The most likely theory of this phenomenon suggests that in the third trimester the lower segment of the uterus lengthens faster than the placenta increases. Thus, a placenta that appears marginal or low-lying at 20 weeks may be normal as the due date approaches. However, the results of most studies of this phenomenon indicate that a complete placenta previa in the second trimester rarely returns to its normal position at term. nine0003

Usually the placenta is relatively homogeneous. Its signal intensity on T1-weighted images is low and slightly higher than that of the myometrium. On T2-weighted images, the placental tissue has a high signal intensity and can be clearly distinguished from the adjacent fetus, uterus, and cervix. Sagittal images best demonstrate the position of the placenta in relation to the internal uterine os. Sometimes endometrial veins can be seen at the edges of the placenta. nine0003

Normal physiological placental calcifications that occur during late pregnancy are usually not seen on MRI.

Placenta previa is diagnosed when placental tissue is found to cover all or part of the internal os of the uterus.

Figure 1 : Sagittal T2-weighted image (SSFSE) showing complete placenta previa at 28 weeks' gestation.

Diagnosis accuracy

No large prospective studies have been conducted to investigate the accuracy of MRI in the diagnosis of placenta previa. However, a series of images taken showed that the results are similar and perhaps slightly better than the results of ultrasound.

The author has not found any reported false negatives in the literature. False-positive results may be due to myometrial contraction in the lower uterine segment on imaging. Although the placental margin remains distinct from the contracted muscle and internal uterine os, the distance between the placental margin and the axis may decrease, leading to a false diagnosis of a low-lying placenta. In extreme cases, the edge of the placenta may come into contact with or even overlap part of the internal os of the uterus and thereby mimic placenta previa. nine0003

Imaging markers

Failure to diagnose placenta previa can have serious consequences during the last trimesters and during labor. The relatively high frequency of placenta previa in the second trimester should not lead to the fact that the doctor cannot diagnose this condition. Whenever a diagnosis is suspected, further evaluation is recommended. In almost all cases, this evaluation includes a repeat ultrasound during the third trimester.

Care should be taken not to confuse a more serious situation such as placental abruption or placenta ingrowth with placenta previa because the treatment of these conditions is different. In addition, the doctor must avoid errors in the search. The possibility of one of these diagnoses complicating placenta previa should be ruled out. nine0003

The time required to organize and conduct an adequate examination may limit its usefulness, especially in the setting of acute maternal bleeding.

At present, the use of MRI to diagnose placenta previa should be limited to a few specific cases, and MRI should be used only after ultrasound has not provided adequate information.

If the internal uterine os can be visualized and if the placental tissue does not overlap it, placenta previa is ruled out. However, an attempt must be made to determine the lowest edge of the placenta and determine the distance between it and the internal axis. When the fetal head obscures the posterior placenta, or when the inferior border of the placenta is not visualized by transabdominal imaging, a transvaginal or transrectal approach is almost always adequate to reveal its position. nine0003

Figure 2 : Longitudinal transabdominal sonogram showing complete symmetrical placenta previa.

Figure 3 : Ultrasound shows an asymmetric complete placenta previa.

Figure 4 : Longitudinal sonogram of the same patient shows that her results are not due to an overdistended bladder, but to true placenta previa. nine0077