How Due Dates Are Calculated (And Why Only 4% Arrive On Time)
Naegele's rule, the cycle-length correction most calculators skip, IVF offsets, and why an early ultrasound overrules your calendar.
A due date is one specific day written on a card, and hardly anyone is born on it. About 4% of babies arrive on the date they were given. The rest show up early, late, or weeks adrift, and everyone involved acts mildly betrayed by the calendar.
The arithmetic behind that date is simpler than people expect. Take the first day of the last menstrual period, add 280 days, and you are done. That’s it. Franz Naegele published the rule in 1812, and obstetrics has been running on it ever since, which means your due date was calculated with a method older than anaesthesia. The Pregnancy Due Date Calculator does the same sum, plus the corrections the plain formula leaves out.
The assumption nobody mentions
Those 280 days hide a guess. The formula assumes you ovulated on day 14 of a 28-day cycle, because that is the textbook cycle. Roughly half of people don’t have one.
Ovulation is tied to the next period, not the last one. It happens about 14 days before bleeding would have started again. So on a 35-day cycle you conceived around a week later than the formula thinks, and your real due date sits a week further out. On a 24-day cycle it moves four days closer. This is the single most useful fix you can apply to a calendar-based estimate, and most online calculators quietly skip it — they ask for your last period and nothing else.
Enter your average cycle length and the adjustment happens automatically.
IVF is the only case with no guesswork
With IVF, conception is known to the hour. Nothing is inferred.
The counting is a bit odd at first. A day-3 embryo transfer already represents 17 days of gestation: 14 days to get to notional ovulation, plus the 3 days the embryo spent in the lab. A day-5 blastocyst is 19 days. From there it’s the usual 280-day count. Clinics do this on paper constantly, and it’s the reason an IVF due date lands within about a day of reality instead of eleven.
Why an early scan beats your calendar
Between 8 and 13 weeks, every fetus is still growing at essentially the same rate. Measure the crown-rump length in that window and you can date the pregnancy to within about five days.
That accuracy doesn’t last. By the second trimester babies have started growing at individual rates, and the same measurement is only good to ten or fourteen days. Which is why ACOG guidance says a date set by a first-trimester scan stands for the rest of the pregnancy, and why clinics replace the period-based date when the two disagree by more than seven days.
If you have a scan report, use the numbers on it. The weeks-and-days figure the sonographer wrote down carries more information than anything your calendar can offer.
What the date is actually for
Not prediction. Scheduling.
The useful range is 37 to 42 weeks, which covers around 80% of spontaneous births. First births skew late: a bit over half happen after the due date. The date exists so the nuchal translucency scan lands in its narrow window, so the glucose test happens before week 28 closes, so anyone discussing induction has a reference point. It’s a coordinate, not a deadline.
Worth knowing too: “12 weeks pregnant” means 12 completed weeks, so you are living through your 13th. The embryo itself is two weeks younger than the number, since conception happened around week 2. That off-by-one gap is why app week counts and books sometimes seem to disagree.
Pick your method, enter one date, and the Pregnancy Due Date Calculator will date every trimester boundary and appointment window for you — in your browser, with nothing sent anywhere.