How to Calculate Your Due Date (and Why It Moves)

Only around one birth in twenty happens on the estimated due date. That single fact tells you most of what you need to know about how to treat it: it is the centre of a range, not an appointment.

Still, it is the number everything else hangs off, so it is worth understanding where it comes from and why it sometimes moves.

Pregnancy is counted from your last period

This is the part that confuses almost everyone. Gestational age is measured from the first day of your last menstrual period, not from conception. It means that at the moment of conception you are already counted as about two weeks pregnant, and that the first two weeks of your "pregnancy" happened before there was a pregnancy.

The reason is practical rather than biological. You are likely to know when your last period started. You are much less likely to know the exact day of conception, which can happen across a window of several days after intercourse. Dating from a known event beats dating from an unknown one.

The three ways a due date gets worked out

1. Naegele's rule, from your last period

Add 280 days to the first day of your last period. That is the classic method, dating from the nineteenth century, and it is still the standard starting point.

The catch is that 280 days assumes a textbook 28-day cycle with ovulation on day 14. If your cycles run to 35 days, you probably ovulate around day 21, and the untouched rule puts your due date a week early. Our due date calculator adjusts for this by adding the difference between your cycle length and 28.

2. From a known conception date

If you know when conception happened, whether through tracking ovulation or through assisted conception, add 266 days. This is the same calculation with the first two weeks removed.

3. From an ultrasound

A dating scan in the first trimester measures the embryo directly, usually crown to rump, and compares it against reference growth. Early in pregnancy, before individual variation has much chance to accumulate, this is accurate to within a few days and is more reliable than any calculation from dates.

Why your due date may change

If a scan date differs from your calculated date by more than about a week, your care team will usually re-date the pregnancy to match the scan. This is routine and not a sign that anything is wrong. Common reasons the original calculation was off:

• Cycles that are longer, shorter, or irregular
• Uncertainty about the date of the last period
• Bleeding in early pregnancy mistaken for a period
• Ovulating earlier or later than usual in that particular cycle

Later scans are less useful for dating, because babies vary more in size as pregnancy progresses. A large baby at 32 weeks is a large baby, not an earlier due date. This is why dating is done early and then left alone.

What the date is actually used for

Beyond the obvious, the due date anchors a lot of scheduling: when screening tests are offered, when a pregnancy counts as term, when induction might be discussed. Getting it right early matters more for those decisions than for knowing which day to pack a bag.

A realistic expectation

Most babies arrive in the two weeks either side of the estimate. A first pregnancy tends to run slightly longer than later ones. Treating the due date as a due month is closer to the truth and considerably less maddening in the final fortnight.

Work out yours with the due date calculator, then see which week you are in.