The most common way to calculate your due date is to count 280 days (40 weeks) from the first day of your last menstrual period. That puts your estimated delivery date about 9 months and 7 days from that starting point. There are a few different ways to get there depending on what information you have, and each one gives a slightly different level of accuracy.
The Standard Formula
The method most doctors use is called Naegele’s Rule, and it works in three simple steps. Take the first day of your last menstrual period, add 7 days, then count back 3 months. So if your last period started on March 1, you’d add 7 days to get March 8, then count back 3 months to land on December 8 as your due date.
This formula assumes a 28-day menstrual cycle, which is important to understand because it changes the math for a lot of people. Ovulation typically happens around day 14 of a 28-day cycle, and that’s the moment conception can actually occur. But the clock starts ticking two weeks before that, on the first day of your period. This means your “gestational age,” the number your doctor uses, is always about two weeks ahead of the actual age of the embryo. When you’re told you’re 6 weeks pregnant, the embryo has really been developing for about 4 weeks.
Adjusting for Your Cycle Length
If your cycle is consistently longer or shorter than 28 days, the standard formula will be off. The fix is straightforward: figure out the difference between your cycle length and 28 days, then add or subtract that number from your due date.
Say your cycle runs 35 days. That’s 7 days longer than 28, which means you likely ovulate around day 21 instead of day 14. Your due date shifts 7 days later than what the standard formula gives you. If your cycle is 25 days, you’d move your due date 3 days earlier. This adjustment matters because the whole formula hinges on when ovulation happened, and cycle length is the best clue you have for that if you weren’t tracking ovulation directly.
For people with very irregular cycles, where the length varies by more than a week from month to month, this adjustment becomes unreliable. In that case, an ultrasound measurement will give a much more accurate estimate.
Calculating From Conception Date
If you know the actual date you conceived, whether from ovulation tracking, timed intercourse, or fertility treatment, the math is even simpler. Add 266 days to the date of conception. That’s 38 weeks, which is the true biological length of pregnancy from fertilization to birth. The reason this number is shorter than the 280 days in the standard method is that you’re skipping those two weeks before ovulation that get counted into gestational age.
For people who underwent IVF, the calculation uses the embryo transfer date. You subtract the age of the embryo at the time of transfer to find the equivalent conception date, then add 266 days. If you had a 5-day embryo transferred on June 20, your conception date would be June 15, and your due date would be 266 days from there: March 8.
How Ultrasound Dating Works
An early ultrasound is the most accurate tool for establishing a due date, especially during the first trimester. The technician measures the embryo from head to rump, a measurement called crown-rump length. Between about 9 and 14 weeks of pregnancy, this measurement reliably predicts gestational age because embryos grow at a very consistent rate during that window. The usable range is roughly 30 to 84 millimeters.
After the first trimester, the head circumference takes over as the primary measurement, usable from about 13 to 30 weeks. But accuracy drops as pregnancy progresses because babies start growing at different rates based on genetics and other factors. A dating ultrasound at 8 weeks is accurate to within a few days. By the second trimester, the margin of error widens to one or two weeks.
If your ultrasound date and your period-based date disagree by more than about a week in the first trimester, your doctor will typically go with the ultrasound. If they’re close, the period-based date usually stands.
Why “Due Date” Is an Estimate
Only about 4 to 5 percent of babies arrive on their actual due date. Full-term birth spans a five-week window, from 37 weeks through 42 weeks, and most babies arrive somewhere in that range. Your due date marks the 40-week midpoint, not a deadline.
Several things can shift when you actually deliver. First pregnancies tend to run a bit longer. The accuracy of your starting information matters too: if you’re unsure of the date of your last period, or if your cycles are unpredictable, the estimate carries more uncertainty from the start. Even with perfect data, natural variation in pregnancy length means the date is always a best guess.
The due date still serves a practical purpose. It’s the anchor point your care provider uses to schedule screenings, blood tests, and growth checks at the right times. It also determines when a pregnancy is considered early term, full term, or overdue, which guides decisions about monitoring and induction.
Quick Reference for Each Method
- Last menstrual period (28-day cycle): First day of last period + 280 days, or add 7 days and subtract 3 months
- Last menstrual period (irregular cycle): Use the standard formula, then add or subtract the difference between your cycle length and 28 days
- Known conception date: Conception date + 266 days
- IVF transfer: Transfer date minus embryo age in days, then + 266 days
- Ultrasound: Measured by your provider using embryo or fetal size, most accurate before 14 weeks