The standard way to figure out your due date is to count 280 days (40 weeks) from the first day of your last menstrual period. That gives you an estimated date, not a guaranteed one. Only about 5% of babies arrive on their exact due date, so think of it as the center of a window rather than a fixed deadline.
The Basic Formula
The most widely used calculation is called Naegele’s Rule, and you can do it without any tools. 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 happening around day 14. It also means there’s a built-in quirk: the first two weeks of your “pregnancy” are counted before conception even happened. Gestational age, the number your provider uses to track your pregnancy, starts from that last period. Your baby’s actual age from conception is roughly two weeks less than whatever gestational age you’re told.
Adjusting for Irregular or Longer Cycles
If your cycle is consistently longer or shorter than 28 days, the standard formula will be off. A 35-day cycle means you likely ovulated around day 21 instead of day 14, pushing your real due date about a week later than Naegele’s Rule would suggest. A 24-day cycle shifts it a few days earlier.
The simplest adjustment is to add 280 days to the first day of your last period, then add or subtract the difference between your average cycle length and 28 days. So for a 35-day cycle, you’d add 7 extra days to the result. If your cycles were highly irregular before pregnancy, use your best estimate of an average cycle length. Your provider will likely rely more heavily on ultrasound to pin things down.
Why Ultrasound Is the Gold Standard
The American College of Obstetricians and Gynecologists considers a first-trimester ultrasound the most accurate method to establish or confirm a due date. During that scan (typically between weeks 8 and 13), the technician measures the baby from head to rump. Early in pregnancy, embryos grow at a remarkably predictable rate, so that single measurement can date a pregnancy to within a few days.
Accuracy drops as the pregnancy progresses. By the second trimester, babies start growing at more individual rates, so measurements become less reliable for dating. Research from the FASTER trial found that when there’s a discrepancy of 9 days or more between period-based dating and a second-trimester ultrasound, the ultrasound is more likely to be correct. Smaller discrepancies generally aren’t worth adjusting for, because the period-based date is just as likely to be right.
If you had a first-trimester ultrasound that disagrees with your period-based date by 7 or more days, your provider will typically switch to the ultrasound date. If the two methods agree within a week, most practices stick with the period-based date.
Due Dates After IVF or Embryo Transfer
If you conceived through IVF, your due date is actually more precise than average because you know exactly when fertilization and transfer happened. There’s no need to estimate ovulation. The calculation starts from the date of your egg retrieval or embryo transfer, depending on whether a fresh or frozen embryo was used.
For a fresh transfer, the retrieval date serves as the equivalent of ovulation day. For a frozen embryo transfer, the transfer date itself anchors the calculation, with the embryo’s age at freezing (day 3, day 5, etc.) factored in. A day-5, day-6, or day-7 frozen embryo all yield the same due date because they’re at the same developmental stage: a blastocyst. Your fertility clinic will provide the specific date, but it’s worth understanding that this estimate carries less uncertainty than a date based on menstrual history alone.
What “40 Weeks” Actually Means
Full-term pregnancy spans a range, not a single point. Your due date marks 40 weeks of gestational age, but babies born anywhere between 39 and 41 weeks are considered full term. Most people go into labor sometime within two weeks on either side of their estimated date. Going a week or even ten days past your due date is common, especially with a first pregnancy.
Because only about 5 in 100 people deliver on the exact predicted day, it helps to think in terms of a “due month” rather than a due date. Planning and expectations feel more realistic when you give yourself that wider window. Providers start monitoring more closely if pregnancy extends beyond 41 weeks, but reaching that point is not unusual.
Quick Ways to Calculate at Home
If you want a fast estimate before your first appointment, you have a few options:
- Naegele’s Rule by hand: First day of last period, minus three months, plus seven days, plus one year.
- The 280-day count: Add 280 days to the first day of your last period. Adjust by the number of days your average cycle deviates from 28.
- Online calculators: Most pregnancy calculators ask for the same two inputs: the first day of your last period and your average cycle length. They’re running the same math described above.
Any of these methods will put you in the right ballpark. Your provider will confirm or adjust the date once you have a first-trimester ultrasound. If you don’t remember the exact start date of your last period, give your best estimate and expect the ultrasound to carry more weight in the final calculation.