How due dates are calculated (and why they move)

Your due date is one of the first numbers you're given in pregnancy, and one of the most misunderstood. Here's how it's worked out, and why it's better thought of as a "due fortnight".

The 280-day count

The traditional method, known as Naegele's rule, estimates your due date as 280 days (40 weeks) from the first day of your last menstrual period (LMP). A quick way to do it by hand: take the first day of your last period, add a year, subtract three months, and add seven days.

It might seem odd to count from your last period when you weren't pregnant yet. The reason is practical: most people know when their last period started, but few know the exact moment of conception. Counting from the LMP gives a consistent starting line.

Why conception sits two weeks in

Because dating starts from your last period, you're considered "two weeks pregnant" at the moment of conception. Ovulation, and therefore conception, typically happens around 14 days after your period begins. That's why a due date based on conception adds 266 days rather than 280. Our conception calculator works this backwards if you already know your due date.

Cycle length matters

The standard count assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, you likely ovulate later or earlier, which nudges your due date. That's why our due date calculator lets you set your cycle length for a more personal estimate.

Dating scans and IVF

The most accurate estimate usually comes from your dating scan at around 12 weeks, which measures the baby and can adjust your due date by a few days either way. If your dates and the scan disagree, the scan normally takes priority.

With IVF, dating is more precise because the timing is known. The due date is calculated from the embryo transfer date and the embryo's age, for example, 261 days from a day-5 blastocyst transfer.

Why so few babies arrive on the day

Only about 1 in 20 babies are born on their actual due date. A first baby often arrives a little after it, and a perfectly healthy pregnancy can deliver any time from 37 to 42 weeks. So it's worth treating your due date as the middle of a range rather than a deadline, and being ready a couple of weeks early, just in case.

This is general information, not medical advice. Your midwife or GP will give you the dates that apply to your pregnancy. See nhs.uk/pregnancy for trusted guidance.