Pregnancy due dates accuracy
How accurate are 'due dates'?
Published
Image source, BBC/ Penelope Chaney/ Eleanor Marshall
By Keith Moore
BBC News
When Penelope Chaney met up with her friend, Eleanor Marshall, she had good news to share - she was pregnant. But Eleanor also had good news... she was pregnant too. And amazingly, both had been given the same predicted delivery date - 4 April. So what are the chances of both giving birth on the same day?
In the UK, where Chaney and Marshall live, a woman's estimated date of delivery is first calculated by using the date of her last period and adding 280 days, or 40 weeks.
That is followed by an ultrasound scan where another estimate is made, based on the size of the foetus.
If the two "due dates" differ by a week or more, the scan is taken as the more accurate measure.
It's the same in most developed countries.
But data from the Perinatal Institute, a non-profit organisation, shows that an estimated date of delivery is rarely accurate - in fact, a baby is born on its predicted due date just 4% of the time.
The figure is higher when premature births and pregnancies with complications are not included, but only marginally (4.4%).
While it may be helpful for parents to get an idea of when their child will arrive, the main purpose of the due date is to "define a metric for the care" of the mother during pregnancy, says Prof Jason Gardosi of the Perinatal Institute.
"So, for example, to interpret early pregnancy blood tests for the risk for example of congenital anomalies, it's important to know how far gone the pregnancy is."
Image source, Thinkstock
The advice to the expectant mother, he says, is that the baby is likely to come any time between 37 weeks (259 days) and 42 weeks (294 days), a period referred to as "term", when the baby has reached full maturity, Gardosi says.
For women like Penelope and Eleanor, whose pregnancies are straightforward and low risk, 60% of babies are born within a week either side of the estimated date of delivery.
More than 90% are born two weeks either side of the predicted date.
But, as noted above, only 4% (or 4.4%, ignoring pregnancies with complications etc) are born on the predicted date itself - in other words, the chance of this happening is less than one in 20.
What, then, are the chances of Penelope's and Eleanor's babies both being born on 4 April?
Only 4.4% of 4.4%, or 0.2%. That's a probability of one in 500.
The chance that their children will be born on the same day - any day, not necessarily 4 April - is higher. It's about one in 30.
Gardosi says the statistics reveal an important message for pregnant women.
The phrase "due date" is misleading. That makes it sound too accurate, he says - it should really be called an "estimated date".
"Many mothers are unnecessarily anxious or impatient if there's too much reliance on the expected date of delivery," Gardosi says.
"We really need to explain to them that this is just a date which helps us to determine the other milestones in pregnancy."
Image source, ALAMY
Estimating delivery
- German obstetrician Franz Karl Naegele (1778-1851) suggested taking the first day of the expectant mother's last period, adding one year, subtracting three months, and adding seven days
- This can vary by up to three days from the usual system used today - adding 280 days (or 40 weeks) to the first day of the last period
- In France it is standard to add two weeks and nine months to the first day of the last period - or a total of 41 weeks - explains Carrieanne Le Bras at the French Mamma website - though this date may be revised in the light of ultrasound tests
- But according to Roshni Patel, an obstetrician at the Chelsea and Westminster Hospital, French doctors "just manage expectations better" - while doctors in the UK or US wait up to two weeks after an estimated delivery date before inducing birth, French doctors give women an extra week to start with, then induce more quickly
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How accurate are your pregnancy due dates?
Throughout my pregnancy, I've received two different due dates — one came from my very first ultrasound, when my baby was barely the size of a blueberry, the date that was then subsequently stamped on my record as "the" due date.
But then there was a second date, the one that showed up on my next couple of ultrasound scans. This date was a few days earlier. It's also the date that I prefer, not only because I'm now 38 weeks pregnant and ready to get this baby out, but because it matched the date I'd calculated on my own using information I looked up.
But ... which one should I believe? And how much does it really matter? After all, as many moms know, due dates are just educated guesses.
How are due dates calculated?
Traditionally, estimated due dates are calculated by taking the start date of a woman's last menstrual period and adding a year, subtracting three months and adding seven days, a method known as Naegele's rule.
"If you're a woman whose period is like clockwork, it's pretty accurate," Dr. Tamika Auguste, MD, an OB-GYN at MedStar Washington Hospital Center in Washington, D.C., told TODAY Health.
But that method doesn't account for irregularities in a woman's menstrual cycle — if she has a longer or shorter cycle than 28 days or any variability with ovulation, for example. That's why the American College of Obstetricians and Gynecologists recommends doctors use ultrasound measurement in the first trimester to establish an estimated due date.
The most accurate way to date a pregnancy is with an ultrasound done between 6 weeks and 8 weeks, 6 days, according to Dr. Michael Cackovic, MD, an OB-GYN with The Ohio State University Wexner Medical Center.
"The reason why is there's very little variation that can be detected in a measurement from that time," he said. "Pretty much all pregnancies are equal at that stage."
As a woman's pregnancy progresses, fetuses develop at different rates, which is why an estimated due date from an ultrasound measurement during the second trimester is less accurate.
(And why I probably shouldn't be taking my second "due date" very seriously.)
As much as technology has improved, this is all still an imperfect science.
"Often we get a due date based on the last menstrual period and then do a sonogram and there's a discrepancy we have to address," Auguste added. "If it's all within a week, it's consistent. Once you go past about five or six days, then you have to start considering that your dates really may be off."
How reliable is my due date?
Even once your doctor has established a due date, there's no guarantee that your baby will come that day. In fact, only 6% of women deliver on their due date, Cackovic said.
I tell patients that really, your due date is a date in the middle of almost a month-long period of time that you could have your baby.
Dr. Michael Cackovic
"The reality is that about 70% of women will have their baby within 10 days of their due date," he said. "I tell patients that really, your due date is a date in the middle of almost a month-long period of time that you could have your baby.
"
Nearly 10% of babies in the U.S. were born before 37 weeks of gestation in 2017, according to data from the Centers for Disease Control and Prevention, while 26% were born between 37 and 38 weeks and 57% were born at full term between 39 and 40 weeks. Some babies arrived after the due date, too: another 6% of babies were born in week 41 and less than 1% at 42 weeks.
Are there signs my baby will come early ... or late?
There are many signs that labor is coming: The cervix softens, a woman may feel that her baby has "dropped" lower in the pelvis or she may experience what's known as "bloody show," discharge that contains mucus and blood.
While these are all signs labor will happen — eventually — there's just no way to know when, exactly.
"The bottom line is we just don't know what starts labor," Cackovic said. "People ask me this all the time. The reality is that it's some complex mechanism between the baby's brain, the mom's brain and the placenta and we haven't figured it out yet.
"
There are some rules of thumb that OB-GYNs follow, though.
"Women who have had multiple babies typically deliver earlier with each subsequent pregnancy," said Dr. Shannon Clark, MD, a professor of maternal-fetal medicine at the University of Texas Medical Branch at Galveston and the founder of BabiesAfter35.com. "First-time moms typically go to their due date in an otherwise uncomplicated pregnancy."
What about routine cervical checks?
Around week 36 or 37 of a woman's pregnancy, some doctors will begin to offer cervical checks at appointments, to see if a woman's cervix has started to dilate (open up) and efface (thin out).
"First-time moms typically efface first, and women who have had more than one delivery typically dilate first," Clark said. "Unfortunately, this does not really tell us when she will go into labor. What we as providers are looking for is some change in the cervix, whether in dilation or effacement, so we will know things are progressing in the right direction.
"
Some doctors may not recommend routine checks, and some women may choose to opt out of them, because while it can be tempting to have the information, being dilated or effaced doesn't tell exactly when the baby's going to come.
"I don't check anybody's cervix unless they ask me to, or if they're contracting," Cackovic said. "Medically, there's just no reason to do it."
While there's no shortage of tips to kick-start labor available on the internet, Auguste stressed that it's important to talk to a doctor about the signs of labor before trying any natural induction methods.
"A lot of the information out there is old wive's tales or hearsay, so you really need to have that conversation with your provider," she said.
How to determine the duration of pregnancy | Center for Fetal Medicine at Chistye Prudy
- Methods for determining the duration of pregnancy
- Embryonic term
- Obstetric term
- By last menstrual period
- By ovulation date or conception date
- According to the size of the uterus
- Ultrasound
- According to the level of hCG in the blood
- According to the first movement of the fetus
Here is a pregnancy test showed the cherished two strips, the expectant mother is in a hurry to register in the antenatal clinic.
The first thing that an obstetrician-gynecologist determines when registering is the gestational age. This indicator is extremely important in order to understand whether the pregnancy is proceeding correctly and the baby is developing, when it is necessary to take tests and undergo additional examinations, when to go on maternity leave and wait for the baby to be born.
It is also very important to know the exact gestational age for screening for the presence of genetic abnormalities in the fetus (ultrasound and blood tests), since these examinations are carried out strictly at certain times of pregnancy.
Turning to an obstetrician-gynecologist, many expectant mothers begin to worry about the difference in terms of pregnancy - what the doctor calculated and the woman herself. In order not to worry in vain, you need to know that there are 2 stages of pregnancy - obstetric and embryonic.
Fetal term
This is the true gestational age from conception, it usually lags behind the obstetric term by about 2 weeks.
Obstetric term
Doctors determine it from the first day of the last menstruation before pregnancy. It should be remembered that all doctors use only the obstetric term, all test results, the size of the fetus, the timing of examinations, maternity leave and the term of delivery are calculated taking into account only the obstetric term of pregnancy.
There are several ways to determine the gestational age.
Determination of the gestational age by the date of the last menstrual period
This is the most common way to calculate the gestational age. However, it can only be used if a woman's menstruation comes regularly at the same interval.
It is not always possible to accurately calculate the duration of pregnancy, guided only by the date of the last menstruation. This happens in cases where a woman has irregular menstruation or in those patients who have a regular but long menstrual cycle. For example, if a woman has a typical cycle length of 35 days (and not 26 - 28, like most women), then most likely she will only be able to conceive on about the 21st day of the cycle (and not on the 14th, as on a 28 day cycle).
Accordingly, the period calculated by menstruation will exceed the “real” obstetric gestational age by a week.
According to the date of ovulation or the date of conception
If the date of conception is known, two weeks must be added to this date - we will get the obstetric gestational age. However, it must be remembered that even if a woman knows exactly the date of ovulation or the date of sexual intercourse, after which pregnancy occurred, this does not mean that she absolutely knows the date of conception.
A spermatozoon that has entered the female body is capable of fertilization within 4-5 days, sometimes even within a week, and a mature egg retains the ability to conceive for 2 days after ovulation. Therefore, even knowing exactly the date of sexual intercourse or ovulation, it is impossible to say with accuracy that fertilization occurred on that day. It could have happened later. Therefore, the period calculated by ovulation or the date of conception cannot be considered completely accurate.
Doctors calculate the gestational age in a slightly different way in cases where pregnancy has occurred as a result of IVF. In this case, the fertilization of the egg by the spermatozoon is carried out "in vitro" by the embryologist. Embryos develop in the laboratory for 3-5 days, after which they are transferred to the uterus.
Doctors calculate the true duration of pregnancy after IVF from the date of ovarian puncture, that is, the stage of the procedure, when the follicular fluid and the follicles contained in it are taken with a special needle for subsequent fertilization “in vitro”, and to determine the “usual” obstetric period , add 2 weeks in the date of ovarian puncture.
If the transfer of the embryo into the uterus was preceded by its cryopreservation (that is, freezing in liquid nitrogen), to determine the exact gestational age, doctors add 5 days to the transfer date (this is the number of days the embryo develops before freezing), and to determine the obstetric period, to the received true date add 2 weeks.
According to the size of the uterus
Examining a woman in a gynecological chair, an obstetrician-gynecologist determines the size of the uterus with both hands. In this case, you can also determine the approximate gestational age.
This method will be most accurate in early pregnancy, up to about 12 weeks. The earliest period that can be determined by the size of the uterus is 5 weeks of pregnancy. By this time, the uterus is slightly enlarged, softened and becomes rounded. At later dates, the size of the uterus may vary slightly depending on the size of the fetus, the amount of amniotic fluid, and the structure of the woman's pelvis. For example, it is believed that at 16 weeks the bottom of the uterus is located in the middle of the distance between the pubis and the navel, at 24 weeks of pregnancy the bottom of the uterus is at the level of the navel.
Ultrasound
In the early stages of pregnancy, by measuring the size of the ovum and embryo, the gestational age can be determined with great accuracy.
At 4-5 weeks, a small “black circle” is detected in the uterus during ultrasound examination - this is a fetal egg, in which an embryo will appear a little later. At about 6-7 weeks, the embryo appears in the form of a small “stripe” and you can see its heartbeat. More accurate is the period calculated by measuring the KTP of the embryo (KTP is the coccygeal-parietal size, that is, the maximum distance from the head end of the fetus to its tailbone), and not by the diameter of the fetal egg.
After 12 weeks, the gestational age during ultrasound is determined according to the so-called fetometry, that is, measurements of various sizes of the head and abdomen of the fetus, length of arms, legs, heart sizes, etc. are used to calculate the term.
Up to 9-10 weeks of pregnancy, the embryo grows proportionally, and its size in all women with the same gestational age will be approximately identical. In the future, the size of the fetus will differ in expectant mothers of different nationalities, with different body weights, the weight of the mother and father at birth will matter, and so on.
That is, in the later stages of pregnancy, normally developing children of the same term may have different sizes (fluctuations can be about 2 weeks, sometimes more), and in such a situation, it is impossible to reliably determine the gestational age according to ultrasound data. In the later stages, the determination of the gestational age by ultrasound is only clarifying. In addition, the lag in the size of the fetus during ultrasound at long gestations is most often regarded by doctors as a developmental disorder due to the fact that the placenta does not transport oxygen and nutrients well enough.
At the Fetal Medicine Center in Moscow, all types of ultrasound during pregnancy are performed at an expert level, including ultrasound in early pregnancy.
Our center is organized in such a way that the whole range of services is concentrated in one place, where a woman receives the results of various types of examinations, including ultrasound, biochemical, and specialist consultation within 1-1.
5 hours.
Blood hCG level
HCG (human chorionic gonadotropin) is a hormone released during pregnancy by the placenta. It begins to be produced with the onset of pregnancy, gradually its amount increases, until about the 11th week of pregnancy, and then begins to decrease slightly.
Determining the concentration of hCG in the blood in the early stages of pregnancy helps to accurately determine the period. Having received the results of a blood test for hCG, it is worth paying attention that in the laboratory tables of the correspondence of the hormone level to the gestational age, the embryonic period is often indicated, that is, to determine the usual obstetric gestational age, 2 weeks should be added to the result.
Recently, tests have appeared to determine the duration of pregnancy by urine. They also determine the concentration of the hCG hormone in the urine of a pregnant woman and, in addition to confirming the very fact of pregnancy, show what period the hormone content corresponds to.
The only thing to remember is that urine tests also show the fetal gestational age.
The Fetal Medicine Center performs all types of tests for pregnant women.
Determining the term of pregnancy by the first movement of the fetus
This method of determining the term has recently been used less and less. It is based on the fact that nulliparous women begin to feel the first movements of the baby at 20 weeks of gestation, multiparous women a little earlier - at 18 weeks. That is why obstetrician-gynecologists recommend that a woman remember the date of the first movement of the fetus and enter this data into the exchange card.
However, this method of determining gestational age is often erroneous.
A mother who is expecting her first child, indeed, most often begins to feel the movements of the fetus a little later than a multiparous woman. This is due to the fact that "experienced" mothers know how the movements of the crumbs are felt at first and what they should feel.
Some primigravidas perceive the first movements of the fetus as an increase in intestinal peristalsis, “gaziki”.
Many women describe the first fetal movements as a feeling of liquid transfusion in the abdomen, "fluttering butterflies" or "swimming fish". The first movements are usually rare, irregular. The time of the first sensations of fetal movements naturally depends on the individual sensitivity of the woman. Some future mothers feel the first movements as early as 15-16 weeks, and someone only after 20. Slender women, as a rule, begin to feel movements earlier than full ones. Women who lead an active lifestyle, work hard, usually feel the movements of the fetus later.
After determining the gestational age, the doctor can calculate the approximate date of delivery. On average, the duration of pregnancy is 280 days or 40 weeks (do not forget that obstetric term is meant).
How to correctly calculate the gestational age and determine the date of delivery
It often comes as a surprise to first-time pregnant women that in obstetrics the gestational age is determined not in months, but in weeks.
But the surprises do not end there - the fact is that the obstetric period is calculated not from conception, but from the first day of the last menstruation.
In fact, pregnancy occurs two weeks after the obstetric period, at the time of ovulation, when the sperm meets the egg. Thus, the age of the embryo, or gestational age, differs from the obstetric one by 2 weeks down.
How long does pregnancy last?
The obstetric term of a term pregnancy is 40 weeks, or 280 days. It is on the basis of the obstetric gestational age that the doctor will prescribe tests and examinations for you, determine the date of maternity leave and calculate the EDD (estimated date of birth).
Doctors use the Negele formula to calculate the EDD. According to this formula, if we add nine months and seven days to the first day of the last menstruation, we get the estimated date of delivery.
Unfortunately, determining the gestational age from the first day of the last menstrual period is not a very accurate method.
It is well suited for women with a stable 28-day cycle, but if your cycle is slightly longer or shorter, then the date of ovulation shifts, respectively, and the actual obstetric gestational age will differ from the established one.
The most accurate EDD can be calculated by adding 266 days to the last ovulation date (if you know it).
How to confirm pregnancy, determine the duration of pregnancy and the date of delivery
By itself, a delay in the start of a new menstrual cycle does not necessarily indicate pregnancy - failures can be explained by diseases, excessive physical exertion or stress. Pregnancy must be confirmed with an hCG test or examination on a gynecological chair.
1) Blood test for hCG
Exceeding the concentration of the "pregnancy hormone" or human chorionic gonadotropin (hCG) in a woman's blood allows diagnosing pregnancy very early, in the first days after its onset, long before a delay in menstruation appears cycle or the woman will feel the first symptoms.
Also, an analysis of the level of hCG in the blood allows you to determine the gestational age with an accuracy of about two weeks.
2) Home pregnancy test
Home pregnancy tests also work by measuring hCG levels, but they are less sensitive than blood tests. The most modern of them can not only confirm the presence of pregnancy a few days before the delay, but also indicate (not too accurately) an approximate date.
Tests of the old generation will show a more or less accurate result only after a delay, that is, after 2-4 weeks from conception.
3) Gynecological examination
A qualified gynecologist-obstetrician can diagnose pregnancy during an examination starting from 3-4 weeks after conception, focusing on changes in the shape and size of the uterus, as well as other signs.
4) Ultrasound
Ultrasound is the most accurate way to diagnose pregnancy. With the help of ultrasound with a transvaginal sensor, it is possible to determine the presence of a fetal egg in the uterus already 1-2 weeks after conception (3-4 obstetric weeks), but fetal heartbeats can only be detected for a period of 5-6 obstetric weeks.
It is possible to determine the gestational age with high accuracy (up to 2-3 days!) With the help of ultrasound only from 6-7 weeks.
If the delay, as well as the result of an hCG test or blood test, indicate that you will soon become a mother, do not rush to get an ultrasound right away. Wait another 2-3 weeks, then by ultrasound you will not only be accurately determined by the period, but will also be allowed to listen to the baby's heartbeat.
If you did not do an early ultrasound to confirm pregnancy, then for the first time you will encounter this study at 10-14 weeks. At the same time, the exact gestational age and PDR will be established or corrected for you. During pregnancy, you will need to undergo such an examination at least twice more. This will happen at 20-24 and 30-34 weeks. However, it is worth saying that ultrasound in the 2nd and 3rd trimesters may have an error in determining the gestational age. The PDR established according to them may differ from the real one by 2-3 weeks.
