The most common way to estimate a due date is to count 280 days (40 weeks) from the first day of your last menstrual period. Only about 5% of babies arrive on that exact date, so think of it as a target window rather than a fixed deadline. Several methods exist to calculate this estimate, and your provider will often use more than one to narrow things down.
The Standard Formula: Naegele’s Rule
The method used most widely in clinical practice is called Naegele’s Rule. Johns Hopkins Medicine breaks it into three steps:
- Find the first day of your last menstrual period (LMP).
- Count back three calendar months from that date.
- Add one year and seven days.
So if your last period started on March 10, you’d count back to December 10, then add a year and seven days to land on December 17 as your estimated due date. The math works out to 280 days, or 40 weeks, from the start of that period.
This formula assumes a 28-day menstrual cycle with ovulation happening around day 14. If your cycles are consistently longer or shorter than 28 days, the estimate shifts. A 35-day cycle, for example, means you likely ovulated a week later than the formula assumes, pushing your realistic due date about a week further out. You can adjust by adding or subtracting the difference between your actual cycle length and 28 days.
How Ultrasound Refines the Estimate
First-trimester ultrasound is the most accurate tool for dating a pregnancy. Between roughly 6 and 14 weeks, a technician measures the embryo from head to tailbone, a measurement called crown-rump length. According to the American College of Obstetricians and Gynecologists (ACOG), this measurement is accurate to within 5 to 7 days.
That precision drops as pregnancy progresses. In the second and third trimesters, babies grow at increasingly individual rates, so size becomes a less reliable indicator of age. This is why an early ultrasound carries the most weight when there’s a conflict between methods. If an early ultrasound date and your LMP-based date disagree by more than a week, providers generally go with the ultrasound.
If you have your first ultrasound later in pregnancy, the margin of error widens to two or even three weeks, making it much harder to pin down a reliable date. Getting an early scan, when possible, gives you the tightest estimate.
Why First-Time Pregnancies May Run Longer
Naegele’s Rule assumes pregnancy lasts 266 days from conception (280 from LMP). But a well-known study published in Obstetrics & Gynecology found that uncomplicated pregnancies tend to run longer than that, and the difference depends on whether you’ve given birth before.
For first-time mothers, the median pregnancy lasted 274 days from ovulation, a full eight days longer than the textbook 266. For those who had given birth before, the median was 269 days, still three days past the standard prediction. Both differences were statistically significant. In practical terms, this means a first-time parent using Naegele’s Rule might reasonably expect delivery closer to 41 weeks than 40.
Calculating a Due Date After IVF
If you conceived through IVF or another assisted reproduction method, the calculation is more straightforward because the conception date is known precisely. The formula starts with your embryo transfer date, subtracts the age of the embryo at transfer, and then adds 266 days.
For a Day 5 embryo transfer on January 20, you’d subtract five days to get a conception date of January 15, then add 266 days. For a Day 3 transfer, you subtract three days instead. Because there’s no guesswork about when fertilization happened, IVF due dates tend to be more accurate from the start than LMP-based estimates.
Fundal Height as a Later Check
Once you’re past 24 weeks, your provider may measure the distance from your pubic bone to the top of your uterus at each visit. This measurement, in centimeters, roughly matches your week of pregnancy, plus or minus about 3 centimeters. At 27 weeks, for instance, the expected measurement is around 27 centimeters.
Fundal height isn’t precise enough to set a due date on its own. It serves as a quick check that growth is tracking as expected. A measurement that’s significantly off might prompt an ultrasound to look more closely at fetal size or amniotic fluid levels, but it won’t change your estimated due date.
What “Due Date” Actually Means
A due date marks the midpoint of a normal delivery window, not a deadline. Full-term birth spans from 37 weeks through 41 weeks and 6 days. Only about 5 in 100 babies arrive on the exact estimated date. Most arrive sometime in the two weeks before or after it.
Several factors influence where in that window your delivery falls. First pregnancies tend to go longer. Shorter menstrual cycles are associated with slightly earlier delivery. Maternal age, ethnicity, and individual biology all play a role. No formula captures all of these variables perfectly, which is why every due date is an estimate.
If you’re trying to plan maternity leave, childcare, or travel, building in a buffer of at least two weeks on either side of your due date gives you the most realistic planning window.