A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. That single date anchors nearly every milestone in prenatal care, from scheduling screenings to tracking fetal growth. The math is straightforward once you know which starting point to use, and there are a few different ways to get there depending on the information you have.
The Standard Formula: Naegele’s Rule
The most widely used method is called Naegele’s Rule, and you can do it without a calculator. Start with the first day of your last menstrual period (LMP), count back three calendar months, then add one year and seven days. If your last period started on June 10, 2025, you’d count back three months to March 10, add a year to get March 10, 2026, then add seven days for a due date of March 17, 2026.
This formula assumes a 28-day menstrual cycle with ovulation happening on day 14. It also counts pregnancy from the start of your last period, not from conception, which means the first two weeks of “pregnancy” technically happen before a fertilized egg even exists. That’s a quirk of the system, but it’s the convention used across obstetric care.
Adjusting for Irregular Cycles
If your cycle is consistently shorter or longer than 28 days, the standard formula will be slightly off. The reason is simple: a longer cycle means you ovulated later than day 14, and a shorter cycle means you ovulated earlier. To adjust, figure out how many days your typical cycle differs from 28, then add or subtract that number from the due date you calculated with Naegele’s Rule.
For example, if your cycle runs 35 days, you ovulate about a week later than the formula assumes. Add seven days to your calculated due date. If your cycle is 24 days, subtract four days. This adjustment only works if your cycles are fairly regular. If your period length varies by more than a week from month to month, your provider will likely rely on an ultrasound to pin down the date.
Calculating From a Known Conception Date
If you know exactly when you conceived (or very close to it), you can add 266 days instead of 280. The 14-day difference accounts for the two weeks between the start of your period and ovulation that Naegele’s Rule includes. Knowing your conception date is uncommon unless you were tracking ovulation closely with tests or temperature charting, but when that information is available, it can be more precise than working from your last period.
How IVF Due Dates Work
For pregnancies conceived through IVF, the calculation starts from the date the embryo was transferred into the uterus. The specific math depends on whether the embryo was transferred at the three-day stage or the five-day (blastocyst) stage, because the embryo’s age at transfer is known precisely. A five-day transfer, for instance, means the embryo is already five days past fertilization, so fewer days are added to reach the 40-week mark. Frozen embryo transfers at the blastocyst stage use the same calculation regardless of whether the embryo was frozen on day five, six, or seven. Because the fertilization date is documented rather than estimated, IVF due dates tend to be the most accurate starting point available.
Why Ultrasound Often Overrides the Math
No matter how carefully you calculate, a first-trimester ultrasound is the most reliable way to confirm or adjust your due date. During the first 13 weeks, the embryo is measured from head to rump (called a crown-rump length), and that measurement predicts gestational age within five to seven days. It’s accurate enough that in one study, 40% of women who received a first-trimester ultrasound had their due date changed because it didn’t match the date calculated from their last period.
The earlier the ultrasound, the tighter the margin. Before nine weeks, a discrepancy of more than five days between the ultrasound and the LMP-based date is enough reason to change the due date. Between nine and fourteen weeks, the threshold shifts to seven days. After the first trimester, ultrasound dating becomes progressively less precise because babies start growing at different rates. That’s why early dating scans carry so much weight.
Once a due date is established, it generally stays put for the rest of the pregnancy. Changing it later can create confusion about whether the baby is growing on track and can affect decisions about timing of delivery.
What “Full Term” Actually Means
Your due date marks 40 weeks, but a healthy delivery doesn’t have to land on that exact day. The American College of Obstetricians and Gynecologists defines “full term” as 39 weeks through 40 weeks and six days. Babies born between 37 and 38 weeks and six days are considered “early term,” not premature, but they do have slightly higher rates of complications than those born after 39 weeks.
Only about 5% of babies arrive on their actual due date. Most are born within a week or two on either side. First-time mothers tend to carry slightly longer than those who’ve given birth before, and individual factors like age and genetics also play a role. The due date is best understood as the center of a window, not a deadline.
Quick Reference for Each Method
- From your last period (28-day cycle): Add 280 days, or count back 3 months and add 7 days plus one year.
- From your last period (non-28-day cycle): Use the same formula, then add or subtract the number of days your cycle differs from 28.
- From a known conception date: Add 266 days.
- From an IVF transfer date: Your clinic will calculate based on whether a day-3 or day-5 embryo was transferred, adjusting for the embryo’s known age.
- From a first-trimester ultrasound: The measurement is converted to a gestational age, which your provider uses to set or adjust the date. This is accurate within five to seven days.