Fact Checked

How do Doctors Predict a Baby's Due Date?

O. Wallace
O. Wallace

One of the most common misconceptions about pregnancy is that it lasts nine months when, in reality, the average pregnancy is 266 days or 38 weeks from conception. Medical professionals typically date a pregnancy in weeks to gain a more accurate and specific age of the pregnancy. They predict a due date by counting back three months from the first day of the last menstrual period (LMP), then adding one year and seven days. Pregnancies are broken up into trimesters: 0-12 weeks make up the first trimester, 13-24 weeks are the second trimester and 25-40 weeks constitute the third trimester.

It was only during the 20th century that predicting a due date became more of a scientific endeavor. While there were probably many “scientific” ways of estimating it in the past, there was no shortage of old wives' methods and incorrect medical theories. A 19th century obstetrician in Germany put forth the idea that a typical pregnancy could be measured by starting with the LMP and counting forward ten moon cycles. This was proven wrong as more modern studies were conducted.

Doctors predicting the due date typically have a margin of error.
Doctors predicting the due date typically have a margin of error.

Recent studies have shown that mothers carrying their first child have pregnancies averaging 41 weeks and one day. For subsequent pregnancies, the average duration is 40 weeks and three days. In general, healthcare professionals agree that all pregnancies, when using the LMP dating technique, average approximately 280 days or 40 weeks, based on a 28 day menstrual cycle. When predicting when the baby is due by using the LMP date, two weeks are added to the fetus’ age, because conception usually occurs two weeks after the start of the LMP. The reason the date is calculated from the LMP is because it is more difficult to pinpoint ovulation and conception than to determine the last menstrual period.

A woman's first pregnancy typically lasts 41 weeks.
A woman's first pregnancy typically lasts 41 weeks.

If a woman has an irregular menstrual cycle or can’t remember when the first day of her LMP was, an ultrasound is performed to help predict the due date. In this procedure, an ultrasound technician measures the fetus to help determine its age. Although an ultrasound performed in the first trimester is considered best for predicting the most accurate date, this method has an error rate of five days on either side. An ultrasound conducted in the second trimester has a plus or minus error rate of eight days, and a third trimester ultrasound has a plus or minus ten day error rate.

Ultrasounds can be used during first trimester to determine due date.
Ultrasounds can be used during first trimester to determine due date.

Even a healthcare professional’s predicted due date is nothing more than a prediction. Only 10% of women deliver on the estimated day, but half of women hit it within one week, and 90% deliver within two weeks of the predicted date. This is why some obstetricians have started predicting due “weeks” instead of days.

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Discussion Comments


The first day of my last period was mid May 2013, making my due week mid February, 20140-- next week! I am having cramps, tightening and pains several times a day, with the frequency increasing.

However the hospital midwives and my OB won't do anything until mid March, as my scans in December, and two this month have measured her at 35 weeks this week? O had no scans before the one in December as I had the summer and autumn from hell with family issues!

Needless to say, I'm frustrated as hell because my cycles have always been like clockwork and I know she was conceived in mid May.


The first day of my last menstrual period was April 26, which would make me 38 weeks, but my baby is measuring out to be 33 weeks. I took four pregnancy tests on May 28, and found out I was pregnant, but the doctors are saying that I got pregnant June 13. How can this be?


I was given a due date of March 1. I had sex May 21 and June 8. Is it possible the conception date could be the May date?


My last period was in February, but my ultrasound is saying that I got pregnant in January. Can it be wrong?


if i had sex with a woman on september 25 and her period was on the 17th or 18th, can she have the baby in july?


I am uncertain of my due date as well. My periods are regular. I mean, very regular. I estimated my due date to be Aug 16th, but my doctor told me an early due date (Aug 9th) based on the ultrasound. Exactly one week early. I am not sure which one to follow.


I went in for my eight week ultrasound and the doctor measured the baby at six weeks. I am confused because my cycles are clockwork and I know when I was ovulating based on my LMP of June 30. Based on my LMP I would be due April 6. Based on doctor's fetal measurements I'm due April 18th. That means I would have conceived days before my missed period. Is that possible?


my daughter in law's midwife told her that she is due in January when my knows her last period started on march 20, giving her a due date of Christmas day, but the midwife says january 11 because of some bone measurement. Just a note here: my kids were both small as I was very small and both of mine were "early".

i was told the following month for both my kids but from my period and my own calendar, I knew when things happened and mine was more accurate too.

Why go by measurements? Don't doctors believe in smaller babies and not go by a normal measurement of a bone? In my opinion, the first day of the menstrual period is the most accurate. Oh. I guess it's because preemies get them more money so maybe that's why it's given a later date.


How accurate is the due date that the doctor gives you?

First, know that only 5 percent of babies are actually born on the predicted due date. An early first trimester ultrasound can predict the due date within 7 days, but still not the exact date.

The health care provider predicts the due date based on when a woman ovulated. However, the date of ovulation can only be ascertained when a women gets her period. ovulation always occurs 14 days before your period arrives.

A lot of good that does, because if you get your period, you are not pregnant. So, the doctor’s predicted date from the pregnancy wheel is only accurate if you have 28 day cycles.

The doctor can and should make adjustments to the due date if you have clockwork cycle length of other durations.

Here’s one example where some more of the inaccuracy comes from: sperm can live up to 72 hours after ejaculation. So, if you have intercourse on cycle day 13, and ovulate on day 14, you may conceive on day 14 or day 15 or day 16 and there is no way of knowing for sure which one.

Another thing to keep in mind is that we don’t really know exactly what really triggers a woman’s body to go into labor. Some women will do strenuous exercise throughout their pregnancies and will have to be induced because they have long passed their due date, while other women are out on bed rest and deliver long before their due date.

Some women go into labor early with little peanut sized babies (like me) and some women hang on till the end with their 9 lb babies.

Remember, babies march to their own drum and arrive on their own schedules! --dearmamadoc


I read this article because my doctor and i cannot agree on one thing: my due date. Honestly, i wouldn't care if there just weren't so much distance between the two due dates. Four weeks. I missed my period in dec 2009 but my doctor thinks that I'm due end of february. i knew at the end of december i was pregnant via pregnancy test and he thinks i have my months mixed up, so i believe this article is accurate and that due dates are questionable.


My last period did not match up with my pregnancy. In truth there should have been one in between my LMP and my conception date so I was dated by ultrasound. But I'm pretty sure they are off by at least four days and up to a week because I had morning sickness days before their estimated date. (And it's my third child. I know when I'm pregnant.)


Okay, so I was taking medication that supposedly messed with my periods. They did a blood test and an ultrasound and said that the age of the baby was 7 weeks and 3 days old. If they went by my last period i would have been 10 weeks and 3 days or so. So if I count back 7 weeks and 3 days, was that the date of conception, more than likely? I've had a nurse tell me this but the more I read things the more I get confused.


nb6300 -- this article confirms the error rate you've read about: 5 days. but, haven't we all heard of babies being 2 weeks late? why wouldn't it be possible that a first trimester ultrasound be 2 to 2 1/2 weeks off? it's just a prediction. plus read the last paragraph of the article: 90% of women deliver within 2 weeks. that means 10% deliver outside of 2 ya, seems like the answer to your question is yes.


Is it possible that a first trimester ultrasound could be 2 - 2.5 weeks off? The error rate I have read about is +/- 5 days.

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