A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. Only about 5% of babies arrive on that exact date, so think of it as an estimate rather than a deadline. There are several ways to calculate it depending on what information you have.
The Last Menstrual Period Method
The most common way to calculate a due date uses a formula called Naegele’s Rule. Johns Hopkins Medicine breaks it into three steps:
- Start with 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, 2025, you’d count back three months to December 10, 2024, then add a year and seven days to get December 17, 2025.
This formula assumes a 28-day menstrual cycle with ovulation happening around day 14. If that describes your cycle, the math works well. If your cycle is significantly shorter or longer, the estimate will be off, sometimes by a week or more.
Adjusting for Irregular Cycles
Naegele’s Rule adds a flat seven days in its calculation, which only works for a textbook 28-day cycle. If your cycles are consistently longer or shorter, you can swap out that number with a personalized one. The adjustment is simple: take your typical cycle length and subtract 21. Use that number in place of the seven days.
For example, if your cycle runs 35 days, you’d subtract 21 to get 14. Instead of adding seven days in the final step, you’d add 14. This pushes your due date out by one week compared to the standard formula, reflecting the fact that you likely ovulate later than day 14. A shorter cycle, say 25 days, would give you a factor of 4, pulling the date earlier.
This adjustment matters most when your cycle regularly differs from 28 days by more than a few days. If your periods are truly unpredictable, varying by weeks from one month to the next, an ultrasound will give you a more reliable estimate than any formula.
Calculating From Conception or Ovulation
If you know the exact date you conceived or ovulated, you can skip the menstrual period math entirely. Add 266 days to the date of conception to get your due date. That’s 38 weeks, which is two weeks shorter than the 40-week count from your LMP, because those first two weeks of “pregnancy” happen before you actually conceive.
Most people don’t know their precise conception date, which is why the LMP method is the default. But if you were tracking ovulation with test strips or temperature charting, that date can be more accurate than a period-based estimate, especially if your cycles are irregular.
Due Dates for IVF Pregnancies
IVF pregnancies have the advantage of a known timeline. The formula is the same 266-day rule, but you first need to calculate the conception date from your transfer. Subtract the embryo’s age at transfer from the transfer date to find the equivalent conception date, then add 266 days.
For a day-5 blastocyst transfer on January 20, for instance, the conception date would be January 15 (five days earlier). Add 266 days to January 15 to get your due date of October 8. For a day-3 embryo, you’d subtract three days instead. Because the dates in IVF are so precisely tracked, due date estimates from this method are among the most accurate available.
How Ultrasound Dating Works
An ultrasound in the first trimester estimates your due date by measuring the baby from head to rump. Before 14 weeks, this measurement is accurate to within five to seven days, making it the most reliable dating tool available at that stage. The earlier the scan, the tighter the accuracy, because embryos grow at nearly identical rates in the first weeks.
Your provider will compare the ultrasound estimate to your LMP-based date. If the two are within a week of each other, your LMP date typically stands. If they differ by more than seven days, the American College of Obstetricians and Gynecologists recommends changing the due date to match the ultrasound. This usually happens when someone has irregular cycles, isn’t sure when their last period started, or ovulated earlier or later than expected.
Later ultrasounds, in the second and third trimesters, are less precise for dating because babies start growing at different rates based on genetics and other factors. That’s why early ultrasound dating carries the most weight.
Why Due Dates Are Estimates
A due date marks the end of 40 weeks of pregnancy, but healthy babies routinely arrive anywhere from 37 to 42 weeks. The 280-day calculation is an average, not a biological rule. Individual variation in when you ovulated, when the embryo implanted, and how quickly it grows all shift the timeline.
First pregnancies tend to run slightly longer. The baby’s sex, your age, and even your own birth history can nudge the timing by a few days in either direction. None of these factors are large enough to formally change your due date, but they help explain why the five-week window around a due date is considered normal.
Your due date matters most as a reference point. It helps your provider track the baby’s growth, schedule routine tests at the right gestational age, and make decisions if the pregnancy goes significantly past 40 weeks. Knowing how it’s calculated, and that it can shift based on better information from an early ultrasound, helps you understand those conversations when they come up.