Home · Guides · How Your Due Date Is Actually Calculated

How Your Due Date Is Actually Calculated — and Why It Moves

6 min read · Updated October 12, 2026 · BabyAwaits editorial

Somewhere on your chart there is a date with a circle around it, and for the next nine months it will quietly organize your life. Have you ever wondered where it actually comes from? The answer involves a formula older than most hospitals, a few assumptions about your cycle, and — often — an ultrasound that quietly overrules both.

The 280-day rule

Nearly every due date starts with Naegele's rule, named for the German obstetrician Franz Naegele, who taught in the early 1800s. The arithmetic is almost embarrassingly simple: take the first day of your last menstrual period, add 280 days, done. That's forty weeks, counted from a period you had before you were pregnant at all.

The reason for the strange accounting is practical. Conception is invisible — most people don't know the moment it happened — but a period leaves a mark on the calendar. So obstetrics counts from the one date almost everyone can pin down. Gestational age, the "weeks and days" your provider speaks in, always includes those first two weeks when there was no baby yet. It's why you're "four weeks pregnant" around the time a test first turns positive.

The textbook version assumes a 28-day cycle with ovulation on day 14. Cycles don't read textbooks. If yours run 32 days, ovulation likely happened four days later than the textbook assumes, and your due date shifts four days with it. Our due date calculator applies exactly this adjustment.

Three dates in, one date out

If you know more than your last period, use it. A known conception date — from tracking ovulation, for instance — gives a cleaner answer: 266 days from conception to due date. IVF dating is cleaner still, because the lab knows the embryo's age to the day. A day-5 blastocyst transfer gets 261 days, a day-3 transfer gets 263, and that figure comes straight from ACOG's Committee Opinion on estimating the due date. If you conceived through IVF, your due date is the most precise kind there is.

Then the ultrasound raises its hand

Here's the part that surprises people: the date from your first-trimester ultrasound beats the date from your calendar. ACOG says so explicitly. Before about 13 weeks, babies grow at nearly the same rate, so a measurement of the embryo — crown to rump, a few millimeters at a time — reads almost like a timestamp. If that measurement disagrees with your LMP date by more than about a week, most providers simply change the due date.

Later ultrasounds are a different story. From the second trimester on, babies grow at genuinely different rates, so a "big" or "small" reading on a 30-week scan says more about the baby's size than about the calendar. That's why a due date rarely changes after the first trimester — and why an ultrasound report that seems off by two weeks at 32 weeks is usually not about dates at all.

It's a due month, really

Only about one pregnancy in twenty delivers on the exact due date. Most babies arrive between 37 and 42 weeks, which is why the honest reading of a due date is a due week — or a due fortnight. First babies average a few days past it; that's biology, not lateness.

Going a week or two past the date is normal enough that providers don't usually intervene right away. What they do instead is watch more closely: check-ups pick up in frequency, and tests track the placenta, which works best before about 42 weeks. If you're staring at a due date that has come and gone, our week 40 and week 41 pages walk through exactly what that extra waiting involves.

What to do with the number

Write it down, tell your family, and then hold it loosely. Use it to pace your maternity leave paperwork and your hospital bag, not as a promise. If you want to see the machinery yourself — forward from your last period, or backward from a date your provider gave you — the due date calculator and the reverse calculator both show their work, using the same formulas your chart does.

Sources: ACOG Committee Opinion 700, Methods for Estimating the Due Date; Naegele's rule as taught in standard obstetric texts.