How is a due date calculated from the last period?

The standard method, Naegele's rule, adds 280 days (40 weeks) to the first day of the last menstrual period. It can also be worked out as: add one year, subtract three months, then add seven days to that same start date — both give the same result.

Why does an ultrasound due date sometimes differ from the period-based one?

Naegele's rule assumes a 28-day cycle with ovulation on day 14, which doesn't hold for everyone. A first-trimester ultrasound measures the fetus directly and is considered more accurate, especially for irregular cycles, so it's typically used to adjust or replace the period-based estimate when the two disagree.

How many babies are actually born on their due date?

Only about 5% of babies are born on their exact estimated due date. Most arrive within roughly two weeks before or after it, which is why a due date is better understood as the middle of a range than a firm deadline.

Does a due date count from ovulation or from the last period?

It counts from the first day of the last menstrual period, which is roughly two weeks before ovulation and conception actually occur in a typical cycle. That's why '40 weeks pregnant' is about 38 weeks of actual fetal development, not 40.

What if I don't know the date of my last period?

This is one of the main situations where a first-trimester ultrasound estimate is used instead of Naegele's rule, since the calendar-based method has no starting point to work from without a known last-period date.

How Pregnancy Due Dates Are Actually Calculated (and Why They're Often Wrong)

Ask where a due date comes from and most people will say "nine months from conception." That's close, but it's not how it's actually calculated. The standard method doesn't start counting from conception at all — it starts from the first day of the last menstrual period (LMP), a date that's usually about two weeks before conception happens. Our Pregnancy Due Date Calculator uses this same standard method; here's the reasoning and the math behind the number it gives you.

Naegele's rule: the actual formula

The method taught in medical training is called Naegele's rule, named for the 19th-century German obstetrician Franz Naegele. It works one of two equivalent ways:

Add 280 days (40 weeks) to the first day of the last menstrual period, or
Add one year, subtract three months, then add seven days to that same start date.

Both land on the same calendar date. For example, an LMP of March 10 gives: add a year → March 10 next year; subtract three months → December 10; add seven days → December 17. That's the estimated due date, 280 days after March 10 of the LMP year.

The number "280 days" itself is really two building blocks stacked together: a 28-day average menstrual cycle, with ovulation assumed to land on day 14 of that cycle, plus a fetal development period of 266 days (38 weeks) counted from conception. Add the 14-day head start to the 266-day count and you get 280 days, or 40 weeks, measured from the LMP — which is why "40 weeks pregnant" actually corresponds to about 38 weeks of real fetal development, not 40.

Why the assumption breaks for a lot of people

Naegele's rule only works cleanly if a cycle is exactly 28 days with ovulation right on day 14 — an average, not a universal rule. Cycles commonly range anywhere from about 21 to 35 days, and ovulation timing shifts along with cycle length. A shorter or longer cycle means conception happened earlier or later than day 14, which shifts the real due date away from what the LMP-only formula produces. The rule also has no way to work at all for anyone who doesn't know or doesn't remember the first day of their last period, or who conceived shortly after stopping hormonal birth control, when normal cycle timing hasn't resumed yet.

Why the ultrasound date can override it

This is why a first-trimester ultrasound, typically performed between about 6 weeks and 8 weeks 6 days of pregnancy, is generally treated as the more reliable estimate when it disagrees with the LMP-based calculation. Early in pregnancy, fetal growth is remarkably consistent from one pregnancy to the next, so measuring the size of the embryo or fetus at this stage gives a tighter estimate of gestational age than a calendar assumption about someone's cycle. If the ultrasound-based date and the LMP-based date differ by more than about a week in the first trimester, standard obstetric guidance is to adjust the due date to match the ultrasound rather than the period-based calculation — the scan effectively corrects for whatever the calendar method got wrong about actual ovulation timing.

Why the date still isn't a deadline

Even a well-calculated due date is an estimate of the middle of a range, not a prediction of the exact day. Only about 5% of babies are born on their actual due date. Most arrive within roughly two weeks before or after it — a spread of about a month wide, centered on the calculated date. That's normal, expected variation, not a sign that the calculation was done wrong. The date is genuinely useful for tracking a pregnancy's progress and scheduling prenatal care around expected milestones; it's just not useful as a hard deadline for when labor "should" start.

Key takeaways

The standard due date counts 280 days (40 weeks) forward from the first day of the last menstrual period — not from conception itself.
Naegele's rule assumes a 28-day cycle with day-14 ovulation, which doesn't match everyone's actual cycle.
A first-trimester ultrasound typically takes precedence over the calendar-based estimate when the two disagree by more than about a week.
Only about 5% of babies are born on their exact due date; a two-week window on either side is normal.

Planning around a due date once you have one is a straightforward date-math problem from there — figuring out how many weeks pregnant a given calendar date represents, or building a simple countdown to the estimated date, both come down to the same day-counting the due date calculation itself relies on.