Your due date is calculated by counting 280 days (40 weeks) from the first day of your last menstrual period. That’s the standard method, and it’s what most online calculators and doctors use as a starting point. But there are several ways to estimate a due date, and each one has a different margin of error depending on how much information you have about your cycle and your pregnancy.
The Last Menstrual Period Method
The most common way to calculate a due date uses a formula called Naegele’s Rule. Here’s how it works: take 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 March 10, you’d count back to December 10, then add a year and seven days to land on December 17 of the following year.
This formula assumes a 28-day menstrual cycle with ovulation happening on day 14. Those assumptions hold true for some people, but not everyone. If your cycles are consistently longer or shorter than 28 days, the estimate shifts. A 35-day cycle, for instance, means you likely ovulated around day 21 instead of day 14, pushing your actual due date about a week later than what the standard formula would suggest. To adjust, simply add the difference between your cycle length and 28 days to the result. If your cycle runs 32 days, add 4 extra days to the standard estimate.
The method also depends on accurately remembering when your last period started. Even being off by a few days changes the calculation, and the formula can’t account for cycles that vary in length from month to month. For people with irregular periods or uncertain dates, other methods become more reliable.
Calculating From Conception Date
If you know exactly when you conceived, the math is simpler: count 266 days (38 weeks) from that date. The two-week difference between this and the 280-day LMP method exists because the LMP count starts about two weeks before conception actually occurs. You’re not technically pregnant during those first two weeks of “pregnancy” by the standard calendar.
Knowing your conception date is most common when you’ve tracked ovulation with test kits or temperature charting, or when conception happened through IVF or another assisted method. If you’re guessing based on when you had sex, keep in mind that sperm can survive for up to five days in the reproductive tract, so the date of intercourse and the date of fertilization aren’t always the same.
How Ultrasound Dating Works
An early ultrasound is the most accurate tool for estimating a due date, and it’s the method most likely to override an LMP-based calculation. Between 7 and 13 weeks, a technician measures the length of the embryo from head to tailbone (called the crown-rump length). At this stage, embryos grow at a remarkably consistent rate regardless of genetics or other factors, making the measurement a reliable indicator of gestational age.
A first-trimester ultrasound is accurate to within 5 to 7 days. That window widens as pregnancy progresses. In the second and third trimesters, measurements of the head, abdomen, and thighbone carry margins of error between 7 and 14 days, because individual growth patterns start to diverge. This is why the dating scan done at your first prenatal visit carries so much weight. If it disagrees with your LMP-based estimate by more than a few days, your provider will typically adjust the official due date to match the ultrasound.
Why Your Due Date Might Change
It’s common to have your due date revised after that first ultrasound, and it doesn’t mean anything is wrong. The LMP method has several built-in weaknesses: it can’t account for inaccurate recall of when your period started, irregular cycle lengths, or variation in when ovulation actually happened. Two people with the same last period date could have conceived a week apart simply because one ovulated earlier than the other.
Doctors generally follow a straightforward principle: the earlier and more precise a dating method is, the more it should be trusted. An LMP that lines up closely with a first-trimester ultrasound gives high confidence in the date. When the two disagree significantly, the ultrasound wins.
How Much Due Dates Actually Vary
Even the most precise due date is still an estimate. A study published through the European Society of Human Reproduction and Embryology tracked pregnancies from the exact date of ovulation and found that the average time from ovulation to birth was 268 days, or 38 weeks and two days. But here’s the striking part: even after excluding preterm births, the natural length of pregnancy varied by as much as 37 days between individuals. That’s more than five weeks of variation among healthy, full-term pregnancies.
This means your due date marks the center of a range, not a deadline. Only about 4 to 5 percent of babies arrive on their exact due date. Most are born within a window of about two weeks on either side. A baby born anywhere between 39 and 41 weeks is considered full term, and providers typically won’t discuss induction until you’ve passed 41 weeks unless there are other concerns.
Quickest Ways to Estimate at Home
If you want a fast estimate before your first appointment, you have a few options:
- 280-day count: Find the first day of your last period on a calendar and count forward 280 days, or 40 weeks. Adjust by adding days if your cycle is longer than 28 days, or subtracting if it’s shorter.
- Naegele’s shortcut: Take your last period date, subtract three months, and add seven days. Same result, less counting.
- From ovulation or conception: If you tracked ovulation, count forward 266 days (38 weeks) from that date.
- Online calculators: Most use the same formulas above. The better ones ask for your cycle length and adjust automatically.
Whichever method you use at home, treat the result as a starting point. Your first ultrasound, ideally before 14 weeks, will either confirm that estimate or give you a more accurate one. The date your provider settles on after that scan is the one used to guide the rest of your care.