A pregnancy due date is calculated by counting 280 days (40 weeks) from the first day of your last menstrual period. That single number drives most of prenatal care, but it’s an estimate, not an expiration date. The average pregnancy actually lasts about 268 days from ovulation, and natural variation means delivery can shift by as much as five weeks in either direction. Here’s how the math works and what can make it more or less accurate.
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. 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 to March 10, then add a year and seven days to land on March 17, 2026.
This formula assumes a 28-day menstrual cycle with ovulation occurring on day 14. It also assumes conception happened about two weeks after your period started, which is why the due date is 40 weeks from your last period rather than 38 weeks from conception. If your cycle runs like clockwork at 28 days, Naegele’s Rule gives a reasonable estimate. If it doesn’t, the date will be off.
Adjusting for Longer or Shorter Cycles
If your cycle is consistently longer or shorter than 28 days, you can correct the formula. The logic is straightforward: a longer cycle means you ovulated later than day 14, so your due date should be pushed forward. A shorter cycle means you ovulated earlier, pulling the due date back.
Take the standard Naegele’s calculation, then add or subtract the number of days your cycle differs from 28. If your cycle is 35 days, add 7 days to the estimated due date. If your cycle is 25 days, subtract 3 days. This correction only works if your cycles are fairly regular. If your period comes anywhere from 25 to 40 days apart with no pattern, a last-period-based calculation isn’t reliable enough to use on its own.
Why Ultrasound Often Overrides the Math
A first-trimester ultrasound is the most accurate way to confirm or adjust a due date. Done before 14 weeks, it measures the embryo from head to rump and estimates gestational age within a window of plus or minus 5 to 7 days. That’s tighter than any formula based on period dates, especially for people with irregular cycles or uncertain recall of their last period.
The American College of Obstetricians and Gynecologists recommends that every pregnancy have an ultrasound before 22 weeks to confirm dating. If the ultrasound-based date differs significantly from the period-based date, your provider will typically update the due date to match the ultrasound. Once a due date is set this way, it rarely changes again. Shifting the due date later in pregnancy can create confusion about whether a baby is preterm or overdue, so any revision is documented carefully and discussed with you directly.
Calculating a Due Date After IVF
If you conceived through in vitro fertilization, the calculation is more precise because the exact date of embryo transfer is known. Your clinic uses the transfer date and the age of the embryo (whether it was transferred at the three-day or five-day stage) to assign a due date. This removes the guesswork about when ovulation happened. Notably, frozen embryo transfers at the blastocyst stage (day 5, 6, or 7) all use the same calculation, since the embryo’s developmental stage at transfer is the same regardless of which specific day it was frozen.
How Precise a Due Date Really Is
The 280-day figure represents an average, not a deadline. Research tracking pregnancies from the confirmed date of ovulation found that the average time from ovulation to birth was 268 days, but individual pregnancies varied naturally by weeks. Whether you’re a first-time parent or have given birth before can also influence timing, as can your age, genetics, and the length of your previous pregnancies.
Because induction is common at or near 40 weeks in the U.S. and other Western countries, it’s difficult to know exactly what percentage of people would naturally deliver on their estimated due date. What’s clear is that most babies arrive within a few weeks of the estimate rather than on the date itself. The due date is best understood as the center of a window, not a target.
Quick Reference for Calculating Your Date
- 28-day cycle: First day of last period minus 3 months, plus 1 year and 7 days.
- Longer cycle (e.g., 32 days): Use the formula above, then add 4 days.
- Shorter cycle (e.g., 25 days): Use the formula above, then subtract 3 days.
- Irregular or unknown cycle: A first-trimester ultrasound provides the most reliable date.
- IVF pregnancy: Your clinic calculates from the embryo transfer date and embryo age.
If you know your last period date and have a regular cycle, the hand calculation gets you close. An early ultrasound confirms or fine-tunes it. Either way, the number you end up with is an educated estimate, and a healthy pregnancy can deliver a week or more on either side of it without anything being wrong.