How pregnancy due dates are calculated
This article is general information, not medical advice. It is not a substitute for a doctor, dietitian or other qualified professional who knows your circumstances. If you have a medical condition, are pregnant, or are changing your diet or exercise significantly, talk to one first.
A due date is calculated by adding 280 days, or 40 weeks, to the first day of your last menstrual period. That is the standard method used before a scan, known as Naegele's rule. It assumes a 28-day cycle with ovulation around day 14, so it is adjusted if your cycles are longer or shorter, or replaced by a calculation from the conception or embryo transfer date if you know it. The due date calculator handles all three, and this guide shows how each one works.
Naegele's rule: 280 days
The rule is named after the German obstetrician Franz Naegele, who described it in the early nineteenth century. The quick version: take the first day of your last period, add one year, subtract three months and add seven days. If your last period started on 10 January, the estimated due date is 17 October. Counting forward 280 days gives the same date, give or take a day in a leap year.
One quirk surprises many people: pregnancy is counted from the last period, not from conception. Conception usually happens about two weeks after the period starts, so at the moment of conception you are already considered two weeks pregnant, and a 40-week pregnancy is about 38 weeks of actual development. This convention, called gestational age, is used because the start of a period is a date most people know, while the day of conception usually is not.
Adjusting for cycle length
Ovulation happens about 14 days before the next period, not 14 days after the last one, so a longer cycle means later ovulation and a later due date. The usual adjustment is to add the difference from 28 days: with a 32-day cycle, add four days; with a 25-day cycle, subtract three. The calculator does this when you change the cycle length. It works best if your cycles are fairly regular. If they vary a lot, or you are unsure of the date of your last period, the period-based estimate is less reliable and the scan matters even more. The fertile window: which days you can get pregnant explains why ovulation is counted back from the next period, and if you logged your cycles in the period tracker, your last period and average cycle length are already recorded.
From conception or IVF
If you know the conception date, the due date is 266 days (38 weeks) later, the same as treating conception as day 14 of a 28-day cycle. IVF dates are the most precise of all, because the age of the embryo is known exactly:
- Day-5 (blastocyst) transfer: due date = transfer date + 261 days.
- Day-3 transfer: due date = transfer date + 263 days.
Fertility clinics usually give you their own date, and the calculator's figure should be within a day or so of it. Whichever method you use, the calculator also shows how many weeks and days pregnant you are today, which trimester you are in and the usual windows for scans and full term.
Why the dating scan wins
Period-based dates rely on memory and on ovulation happening on schedule, and either can be off. In England a dating scan is offered at around 10 to 14 weeks. It measures the baby from crown to rump, and because babies grow at a very similar rate in early pregnancy, this predicts gestational age to within a few days. If the scan and the period-based date differ by more than a small margin, the scan date is normally used, and that becomes the date your care is planned around. Later scans are less useful for dating because babies' sizes vary more as pregnancy goes on. The date difference calculator is handy for counting the weeks between appointments.
When babies actually arrive
A due date is the middle of a range, not an appointment. Only about 4 to 5% of babies are born on their due date, and most arrive within two weeks either side of it. A pregnancy is considered term from 37 weeks to 42 weeks. If labour has not started by around 41 weeks, induction is usually discussed; your midwife or doctor will explain the options and the reasons for them. Contact your maternity unit straight away if you have vaginal bleeding, your waters break, your baby's movements slow down or change, or you have regular contractions before 37 weeks. When the time comes, the contraction timer records how long your contractions last and how far apart they are. This guide is general information, not medical advice.
Sources and further reading
The claims in this guide rest on these references, which were checked when the guide was last updated. Spotted an error? The contact page says how to report it.