How to calculate due date without a period
How to Calculate Your Due Date
Pregnancy lasts an average of 280 days (40 weeks) from the first day of your last menstrual period (LMP). The first day of your LMP is considered day one of pregnancy, even though you probably didn’t conceive until about two weeks later (fetal development lags two weeks behind your pregnancy dates).
Read our report on the 13 best pregnancy iPhone and Android apps of the year here.
Calculating your due date isn’t an exact science. Very few women actually deliver on their due date, so, while it’s important to have an idea of when your baby will be born, try not to get too attached to the exact date.
If you have regular 28-day menstrual cycles, there are two ways to calculate your due date.
Naegele’s rule
Naegele’s rule involves a simple calculation: Add seven days to the first day of your LMP and then subtract three months.
For example, if your LMP was November 1, 2017:
- Add seven days (November 8, 2017).
- Subtract three months (August 8, 2017).
- Change the year, if necessary (to the year 2018, in this case).
In this example, the due date would be August 8, 2018.
Pregnancy wheel
The other way to calculate your due date is to use a pregnancy wheel. This is the method that most doctors use. It’s very easy to estimate your due date if you have access to a pregnancy wheel.
The first step is locating the date of your LMP on the wheel. When you line up that date with the indicator, the wheel displays your due date.
Remember that the due date is only an estimate of when you will deliver your baby. The chances of actually having your baby on that exact date are very slim.
This is more common than you’d think. Luckily, there are ways to figure out your due date when you can’t remember the first day of your LMP:
- If you know you had your LMP during a particular week, your doctor can estimate your due date accordingly.
- If you have no idea when your last period was, your doctor may order an ultrasound to determine your due date.
Some women have cycles that are consistently longer than the average 28-day cycle. In these cases, a pregnancy wheel can still be used, but some simple calculations are necessary.
The second half of a woman’s menstrual cycle always lasts for 14 days. This is the time from ovulation to the next menstrual period. If your cycle is 35 days long, for example, then you probably ovulated on day 21.
Once you have a general idea of when you ovulated, you can use an adjusted LMP to find your due date with a pregnancy wheel.
For example, if your menstrual cycle is usually 35 days long and the first day of your LMP was November 1:
- Add 21 days (November 22).
- Subtract 14 days to find your adjusted LMP date (November 8).
After you calculate your adjusted LMP date, simply mark it on the pregnancy wheel and then look at the date where the line crosses. That is your estimated due date.
Some pregnancy wheels may allow you to enter the date of conception — which occurs within 72 hours of ovulation — instead of the date of your LMP.
Your doctor may change your due date if your fetus is significantly smaller or larger than the average fetus at your particular stage of pregnancy.
Generally, your doctor orders an ultrasound to determine the gestational age of your baby when there’s a history of irregular periods, when the date of your LMP is uncertain, or when conception occurred despite oral contraceptive use.
An ultrasound allows your doctor to measure the crown-rump length (CRL) — the length of the fetus from one end to the other.
During the first trimester, this measurement provides the most accurate estimation for the age of the baby. Your doctor may change your due date based on the ultrasound measurement.
This is most likely to occur in the first trimester, especially if the date estimated by the ultrasound differs by more than one week from the date estimated by your doctor based on your LMP.
In the second trimester, an ultrasound is less accurate and your doctor probably won’t adjust your date unless the estimates vary by more than two weeks.
The third trimester is the least accurate time to date a pregnancy. Estimates based on an ultrasound can be off by as much as three weeks, so doctors rarely adjust dates during the third trimester.
However, it’s not uncommon for a doctor to perform an ultrasound in the third trimester if they’re thinking about changing your date.
A repeat ultrasound provides valuable information about the growth of the fetus and may reassure you and your doctor that the change in due date is reasonable.
Did you know?Ultrasound measurements for estimating the age of a fetus are more accurate during the early stages of pregnancy. In the first few weeks, fetuses tend to develop at the same rate. However, as pregnancy progresses, the rates of fetal growth begin to vary from pregnancy to pregnancy.
This is why ultrasound measurements can’t be used to accurately predict the age of the baby in the later stages of pregnancy.
Ultrasounds are not a necessary part of prenatal care.
Avoid numerous scansTrusted Source and have ultrasounds for medical reasons only.
When a doctor performs an ultrasound, they write a report on the findings and include two estimated due dates. The first date is calculated using the date of the LMP. The second date is based on the ultrasound measurements. These dates are rarely the same.
When your doctor evaluates the ultrasound results, they’ll determine whether or not these dates are in agreement. Your doctor probably won’t change your due date unless its significantly different from your ultrasound date.
If you have more ultrasounds, each ultrasound report will contain a new due date based on the most recent measurements. An expected due date shouldn’t be changed based on measurements from a second- or third-trimester ultrasound.
Due date estimations are more accurate earlier in pregnancy. Later ultrasounds are helpful in determining whether the fetus is growing well but not for determining the age of the fetus.
Learn more about how your body changes during your pregnancy.
- ACOG reinvents the pregnancy wheel: Launches new due date app. (2016). https://www.acog.org/About-ACOG/News-Room/News-Releases/2016/ACOG-Reinvents-the-Pregnancy-Wheel
- Avoid fetal “keepsake” images, heartbeat monitors. (2014). https://www.fda.gov/ForConsumers/ConsumerUpdates/ucm095508.htmTrusted Source
- Calculating a due date. (n.d.). http://www.hopkinsmedicine.org/healthlibrary/conditions/pregnancy_and_childbirth/calculating_a_due_date_85,P01209/
- Calculating your estimated due date. (2014). https://my.clevelandclinic.org/health/diseases_conditions/hic_Am_I_Pregnant/hic_Calculating_Your_Estimated_Due_Date
- Due date calculator. (n.d.). http://www.marchofdimes.org/pregnancy/calculating-your-due-date.aspx
- Pregnancy due date and gestational age calculator. (n.d.). http://www.perinatology.com/calculators/Due-Date.htm
How to Calculate Your Due Date
Pregnancy lasts an average of 280 days (40 weeks) from the first day of your last menstrual period (LMP).
The first day of your LMP is considered day one of pregnancy, even though you probably didn’t conceive until about two weeks later (fetal development lags two weeks behind your pregnancy dates).
Read our report on the 13 best pregnancy iPhone and Android apps of the year here.
Calculating your due date isn’t an exact science. Very few women actually deliver on their due date, so, while it’s important to have an idea of when your baby will be born, try not to get too attached to the exact date.
If you have regular 28-day menstrual cycles, there are two ways to calculate your due date.
Naegele’s rule
Naegele’s rule involves a simple calculation: Add seven days to the first day of your LMP and then subtract three months.
For example, if your LMP was November 1, 2017:
- Add seven days (November 8, 2017).
- Subtract three months (August 8, 2017).
- Change the year, if necessary (to the year 2018, in this case).
In this example, the due date would be August 8, 2018.
Pregnancy wheel
The other way to calculate your due date is to use a pregnancy wheel. This is the method that most doctors use. It’s very easy to estimate your due date if you have access to a pregnancy wheel.
The first step is locating the date of your LMP on the wheel. When you line up that date with the indicator, the wheel displays your due date.
Remember that the due date is only an estimate of when you will deliver your baby. The chances of actually having your baby on that exact date are very slim.
This is more common than you’d think. Luckily, there are ways to figure out your due date when you can’t remember the first day of your LMP:
- If you know you had your LMP during a particular week, your doctor can estimate your due date accordingly.
- If you have no idea when your last period was, your doctor may order an ultrasound to determine your due date.
Some women have cycles that are consistently longer than the average 28-day cycle.
In these cases, a pregnancy wheel can still be used, but some simple calculations are necessary.
The second half of a woman’s menstrual cycle always lasts for 14 days. This is the time from ovulation to the next menstrual period. If your cycle is 35 days long, for example, then you probably ovulated on day 21.
Once you have a general idea of when you ovulated, you can use an adjusted LMP to find your due date with a pregnancy wheel.
For example, if your menstrual cycle is usually 35 days long and the first day of your LMP was November 1:
- Add 21 days (November 22).
- Subtract 14 days to find your adjusted LMP date (November 8).
After you calculate your adjusted LMP date, simply mark it on the pregnancy wheel and then look at the date where the line crosses. That is your estimated due date.
Some pregnancy wheels may allow you to enter the date of conception — which occurs within 72 hours of ovulation — instead of the date of your LMP.
Your doctor may change your due date if your fetus is significantly smaller or larger than the average fetus at your particular stage of pregnancy.
Generally, your doctor orders an ultrasound to determine the gestational age of your baby when there’s a history of irregular periods, when the date of your LMP is uncertain, or when conception occurred despite oral contraceptive use.
An ultrasound allows your doctor to measure the crown-rump length (CRL) — the length of the fetus from one end to the other.
During the first trimester, this measurement provides the most accurate estimation for the age of the baby. Your doctor may change your due date based on the ultrasound measurement.
This is most likely to occur in the first trimester, especially if the date estimated by the ultrasound differs by more than one week from the date estimated by your doctor based on your LMP.
In the second trimester, an ultrasound is less accurate and your doctor probably won’t adjust your date unless the estimates vary by more than two weeks.
The third trimester is the least accurate time to date a pregnancy. Estimates based on an ultrasound can be off by as much as three weeks, so doctors rarely adjust dates during the third trimester.
However, it’s not uncommon for a doctor to perform an ultrasound in the third trimester if they’re thinking about changing your date.
A repeat ultrasound provides valuable information about the growth of the fetus and may reassure you and your doctor that the change in due date is reasonable.
Did you know?Ultrasound measurements for estimating the age of a fetus are more accurate during the early stages of pregnancy. In the first few weeks, fetuses tend to develop at the same rate. However, as pregnancy progresses, the rates of fetal growth begin to vary from pregnancy to pregnancy.
This is why ultrasound measurements can’t be used to accurately predict the age of the baby in the later stages of pregnancy.
Ultrasounds are not a necessary part of prenatal care. Avoid numerous scansTrusted Source and have ultrasounds for medical reasons only.
When a doctor performs an ultrasound, they write a report on the findings and include two estimated due dates.
The first date is calculated using the date of the LMP. The second date is based on the ultrasound measurements. These dates are rarely the same.
When your doctor evaluates the ultrasound results, they’ll determine whether or not these dates are in agreement. Your doctor probably won’t change your due date unless its significantly different from your ultrasound date.
If you have more ultrasounds, each ultrasound report will contain a new due date based on the most recent measurements. An expected due date shouldn’t be changed based on measurements from a second- or third-trimester ultrasound.
Due date estimations are more accurate earlier in pregnancy. Later ultrasounds are helpful in determining whether the fetus is growing well but not for determining the age of the fetus.
Learn more about how your body changes during your pregnancy.
- ACOG reinvents the pregnancy wheel: Launches new due date app. (2016). https://www.acog.org/About-ACOG/News-Room/News-Releases/2016/ACOG-Reinvents-the-Pregnancy-Wheel
- Avoid fetal “keepsake” images, heartbeat monitors.
(2014). https://www.fda.gov/ForConsumers/ConsumerUpdates/ucm095508.htmTrusted Source - Calculating a due date. (n.d.). http://www.hopkinsmedicine.org/healthlibrary/conditions/pregnancy_and_childbirth/calculating_a_due_date_85,P01209/
- Calculating your estimated due date. (2014). https://my.clevelandclinic.org/health/diseases_conditions/hic_Am_I_Pregnant/hic_Calculating_Your_Estimated_Due_Date
- Due date calculator. (n.d.). http://www.marchofdimes.org/pregnancy/calculating-your-due-date.aspx
- Pregnancy due date and gestational age calculator. (n.d.). http://www.perinatology.com/calculators/Due-Date.htm
How to calculate the gestational age - Lifehacker
December 27, 2022 Likbez Health
This task is best handled by a doctor.
Determining the exact gestational age is not an easy task. It is not enough to know when sexual intercourse occurred. In order not to be mistaken, one must be aware of the woman's monthly cycle, as well as take into account dozens of other factors, such as the size of the uterus and embryo.
The most reliable way is to see a gynecologist, answer his questions, undergo a physical examination and do an ultrasound. Based on the results of the examination, you will be told the duration of your pregnancy.
However, you can more or less accurately estimate what week you are in on your own. But for this you need to know some nuances.
Why there is confusion about the timing of pregnancy
Suppose you know for sure that the conception took place only two weeks ago and not a day earlier. However, the doctor you contacted about the delay in menstruation, after ultrasound and other events, sets a different gestational age - 4-5 weeks. Who is right? Basically, both sides. But the gynecologist is still right.
Many pregnant women face this confusion.
The fact is that the expectant mother is trying to calculate the gestational age in real, so-called embryonic weeks. A gynecologist - in obstetrics. The difference between them is approximately 14 days.
Obstetric weeks are considered a more accurate method of measuring pregnancy. And there are good reasons for that.
What are fetal weeks and why they are not accurate
Embryonic weeks count the actual age of the fetus - that is, the period that has passed since conception. At first glance, this looks like the most logical option for determining the gestational age. But there are two problems.
1. The day of sexual intercourse cannot serve as a starting point
Fertilization occurs only at ovulation - a short period, about a day, when the egg is released from the ovary. If at this time she meets with sperm, conception will occur. If not, you will have to wait for the next cycle.
It is believed that in a 28-day cycle, ovulation occurs on the 14th day. However, the dates are floating: the egg can leave the ovary a few days earlier, and a couple of days later than the estimated date.
Spermatozoa, in turn, are able to wait for the egg in the fallopian tube for 2-4 days.
This means that fertilization does not occur at the time of sexual intercourse, and sometimes only a few days later.
2. Pregnancy does not occur when the egg is fertilized by the sperm
Conception occurs only when the fertilized egg is implanted in the wall of the uterus. By the way, this doesn't always happen. If a woman takes oral contraceptives or, for example, something is wrong with the embryo, she will not become pregnant.
Implantation of the egg, that is, the immediate start of pregnancy, occurs on average 6 days after fertilization.
Therefore, it is impossible to accurately determine the physical gestational age and embryonic age of the fetus. To avoid inaccuracies, the period is calculated in obstetric weeks.
How obstetric weeks are calculated
They are calculated not from the moment of conception, but from the first day of the last menstruation. As a rule, all women know exactly this date, so mistakes are almost impossible.
Obstetric pregnancy is on average 14 days longer than what a woman considers real.
This difference is equal to the average period between the first day of menstruation and expected ovulation.
However, sometimes this method of calculation fails. Therefore, gynecologists determine the gestational age by the last menstruation, and then compare it with ultrasound data at the 11-14th week. This is because in the first trimester, the embryo develops at the same rate in all women, so its size is standard. They can be easily measured during an ultrasound examination.
If the discrepancy is more than 5 days, consider that the correct date was shown by ultrasound.
Why do you need to count obstetric weeks
Firstly, each of them is associated with one or another important stage in the development of the unborn baby.
Secondly, the date of the onset of menstruation is closely related not only to ovulation, but also to the expected date of delivery (ED).
To calculate the EDD, gynecologists use the so-called Naegele's rule, according to which exactly 40 obstetric weeks or 280 days pass between the first day of menstruation and childbirth.
The calculation according to the Naegele rule takes place in three stages.
- The first day of the last menstrual cycle is determined.
- Exactly three calendar months are subtracted from this date.
- The year and 7 days are added to the received date.
For example, a woman's last period began on November 9, 2022. If you subtract 3 calendar months, it will be released on August 9, 2022. We add the year and 7 days and get August 16, 2023. This date will be the expected date of birth.
How to calculate the gestational age
It's simple. Mark your last period on your calendar and count how many weeks have passed since then.
If, for example, your last period started on November 15 and today is December 27, you are 6 weeks pregnant.
What applications will help you calculate the gestational age
In order not to calculate the weeks every time again, you can use mobile organizer applications. It is enough to enter the date of the first day of the last menstruation once - and the program will automatically calculate which obstetric week you are in. At the same time, it will help you track your well-being, remind you of the need for scheduled visits to the doctor and give information about the development of the baby.
1. Pregnancy Calendar
The application helps to accurately calculate and track the obstetric gestational age, and also determines the expected date of delivery. In addition, it tells about how the child grows, what happens to him in each particular week. This information is presented in simple and visual images: for example, the weight of a baby is compared with the mass of popular vegetables and fruits.
The mother's condition is monitored separately. At the start, the application will ask you to specify individual characteristics - age, height, weight.
Based on these data, the program calculates the optimal weight gain and gives recommendations on how to take care of yourself, what to eat and what exercises to do.
Download
Price: Free
Download
Price: Free
2. Pregnancy Tracker
This utility is created by BabyCenter, the world's largest online resource for motherhood and childhood. Leading doctors of the best American clinics act as consultants in it.
In addition to calculating your obstetric gestational age and expected due date, you can also get expert information on how your baby develops day by day. The mom-to-be will receive advice on nutrition, vitamins, exercise, as well as weekly checklists with reminders to visit the gynecologist or take routine tests.
The only drawback of the application is that it is in English.
Download
Price: Free
Download
Price: Free
3.
Mom Life happens to her and the baby every week.
The feature of this utility is social contacts. By providing Mom Life with your location, you can find women of the same gestational age who live near you. Also in the application are available reviews about doctors, clinics, maternity hospitals located nearby.
There is also a global chat where you can communicate with mothers from different countries and, if necessary, receive advice and support from them.
Download
Price: Free
Download
Price: Free
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For first-time pregnant women, it often comes as a surprise that in obstetrics, 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 start of 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 birth.
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 way to calculate the EDD is to add 266 days to the date of the last ovulation (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 the blood of a woman 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 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 the 10-14th week. 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.
