How Pregnancy Due Dates Are Calculated
If you're expecting, one of the first questions is: when is the baby due? The estimated due date (EDD) gives you a target to plan around. Here's how that date is worked out and why it's an estimate, not a guarantee.
The standard method (Naegele's rule)
The most common method counts from the first day of your last menstrual period (LMP). You take that date, add one year, subtract three months, and add seven days — which works out to about 40 weeks (280 days) from your LMP. Most due-date calculators do exactly this for you.
Why 40 weeks?
Pregnancy is measured from the LMP rather than conception because the LMP is a date most people can identify, while the exact day of conception usually isn't known. Since ovulation typically happens about two weeks after the LMP, the actual time from conception is closer to 38 weeks — but the 40-week convention keeps everyone using the same reference point.
Why it's only an estimate
- Cycle length varies — calculations assume a regular 28-day cycle, so longer or shorter cycles shift the date.
- Only about 1 in 20 babies arrive exactly on the due date.
- Most births happen within a window of about two weeks either side.
- An early ultrasound often gives a more accurate date than the LMP method.
Due date vs conception date vs ultrasound dating
Three different dates float around in early pregnancy and they're easy to muddle. The LMP-based due date is the planning standard. The conception date — roughly two weeks after the LMP for a typical cycle — is when pregnancy actually began, which is why 'weeks pregnant' counted from the LMP always runs about two ahead of weeks-since-conception. And a dating ultrasound, especially in the first trimester, measures the baby directly and is often the most accurate of the three; if it disagrees with the LMP date by more than a few days, providers usually go with the scan.
What the weeks and trimesters mean
The 40 weeks split into three trimesters: roughly weeks 1–12, 13–27 and 28–40. Knowing your week helps you follow what's developing and when checkups and screenings tend to be scheduled. A baby is considered 'full term' from 37 weeks, which is the real reason to treat the due date as the middle of a normal arrival window rather than a hard deadline circled on the calendar.
What happens if you go past your due date?
Plenty of pregnancies sail past the estimated date, and on its own that is usually nothing to worry about — remember only about one baby in twenty arrives on the day itself. What changes is the level of attention. As you move beyond 40 weeks, providers typically schedule more frequent checks, monitoring the baby's heartbeat and the amniotic fluid to make sure all remains well. If the pregnancy continues past around 41 to 42 weeks, many providers will discuss inducing labour, because the risks gradually rise once a pregnancy becomes 'post-term'. The takeaway is simple: a date that comes and goes is normal, but your appointments become more important, not less, in the days afterwards.
Why twins and IVF pregnancies are dated differently
The standard last-period method assumes a single baby conceived naturally on a regular cycle, so two common situations are handled differently. With IVF, the date of egg retrieval or embryo transfer is known precisely, so the due date is calculated from that exact point rather than from a last period — usually making it more accurate than any estimate. Twins and other multiples, meanwhile, are very often born earlier than 40 weeks, so while the due date is still calculated the same way, your provider will plan around an earlier likely arrival. In both cases, your care team's dating takes precedence over any general calculator.
Frequently asked questions
Can my due date change during pregnancy?
Yes. An early ultrasound can adjust it if it differs noticeably from the LMP estimate, and providers generally trust an early scan over the LMP date because it measures the baby directly.
What if I don't know my last period date, or have irregular cycles?
Then the LMP method is unreliable and a dating ultrasound becomes the main way to estimate the due date. Irregular cycles also shift ovulation, which the standard 28-day assumption simply can't account for.
Does the due date predict the exact day of birth?
No — only about 1 in 20 babies actually arrive on it. Most come within roughly two weeks either side, so treat it as a target window and keep your plans flexible as you near the end.
The bottom line
A due date is a useful planning estimate calculated as roughly 40 weeks from your last period — but think of it as a window, not a deadline. Use our calculator for a quick estimate, and always confirm with your doctor or midwife.