Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. That’s the standard method, and it’s what most online calculators and doctors’ offices use as a starting point. But the math shifts depending on your cycle length, whether you know your conception date, and what an ultrasound shows.
The Standard Formula: Naegele’s Rule
The most common way to estimate a due date is a three-step formula called Naegele’s Rule, outlined by Johns Hopkins Medicine:
- Start with the first day of your last menstrual period (LMP).
- Count back three calendar months.
- Add one year and seven days.
So if your last period started on March 10, you’d count back to December 10, then add a year and seven days, giving you a due date of December 17. This formula assumes a 28-day menstrual cycle and that ovulation happened on day 14. If that matches your body, the estimate is fairly reliable. If your cycles are longer or shorter, you’ll need to adjust.
Adjusting for Irregular or Longer Cycles
Naegele’s Rule bakes in the assumption that you ovulated 14 days after your period started. But if your cycle is 35 days, you likely ovulated around day 21, not day 14. That seven-day difference means your due date would be about a week later than the standard formula suggests.
The adjustment is straightforward: take the difference between your actual cycle length and 28 days, then add that number to your Naegele’s Rule result. A 32-day cycle adds 4 days. A 24-day cycle subtracts 4 days. This only works if your cycles are reasonably consistent. If your period is unpredictable from month to month, the LMP-based calculation becomes much less accurate, and ultrasound dating becomes more important.
Calculating From a Known Conception Date
If you know the exact date of conception, the math is simpler: add 266 days to that date. That’s 38 weeks, which is the actual length of fetal development from fertilization to delivery. The reason the LMP method uses 280 days (40 weeks) instead is that it counts two extra weeks before conception, back to when your period started.
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 tests or temperature charting and had a narrow window of intercourse, the conception-based method can be more precise.
Due Date Calculation for IVF
IVF pregnancies have the advantage of a known timeline. The calculation works backward from the embryo transfer date. First, subtract the embryo’s age at transfer: 3 days for a cleavage-stage embryo, or 5 to 6 days for a blastocyst. That gives you the equivalent conception date. Then add 266 days to get the due date.
For example, if you had a day-5 blastocyst transferred on June 20, your conception date equivalent would be June 15. Adding 266 days puts your due date around March 8. This method is considered highly accurate because there’s no guesswork about when fertilization happened.
How Ultrasound Changes the Estimate
Regardless of how you calculate your due date at home, your doctor will likely adjust it based on an ultrasound. The American College of Obstetricians and Gynecologists considers first-trimester ultrasound the most accurate method for establishing a due date. Early in pregnancy, embryos grow at nearly identical rates, so measuring the embryo’s size gives a reliable estimate of gestational age.
If the ultrasound date and your LMP-based date are close, your original estimate usually stands. But if there’s a meaningful gap, the ultrasound wins. The thresholds for when doctors revise the due date depend on timing: earlier ultrasounds are more accurate, and discrepancies of more than a few days in the first trimester are enough to trigger a change. After about 22 weeks, ultrasound measurements become less precise because babies start growing at different rates, and dating from that point is considered suboptimal.
This is why the recommended window for a dating ultrasound is 18 to 22 weeks if you haven’t had an earlier scan. That window balances accurate dating with a detailed look at fetal anatomy.
Why Due Dates Are Estimates
Only about 4% of babies arrive on their exact due date. Pregnancy length varies naturally from person to person. Women who have given birth before tend to deliver about 3 days earlier than first-time mothers. Maternal age plays a role too: both younger and older women tend to have slightly shorter pregnancies compared to those in the middle range.
Rather than a single date, it helps to think of your due date as the center of a window. Pregnancy is now classified into specific ranges: early term covers 37 weeks through 38 weeks and 6 days, full term starts at 39 weeks, and postterm begins at 42 weeks. A baby born anywhere in the full-term window is considered right on time, even if it’s a week before or after the number circled on your calendar.
Quick Reference for Each Method
- From your last period (28-day cycle): First day of LMP + 280 days, or count back 3 months and add 7 days plus one year.
- From your last period (irregular cycle): Use the formula above, then add or subtract the difference between your cycle length and 28 days.
- From conception date: Conception date + 266 days.
- From IVF transfer: Transfer date minus embryo age in days, then + 266 days.
Any of these methods gets you a reasonable starting estimate. Your provider will refine it with ultrasound measurements, and that adjusted date is the one used to guide your care throughout pregnancy.