Pregnancy Due Date Calculator

Naegele’s rule, and why the cycle length matters

The rule is 280 days - 40 weeks - from the first day of the last period. It works because a pregnancy is dated from the last period rather than from conception: nobody knows the day of conception, and everybody can name the first day of a bleed, so the convention is the one that can be applied consistently.

That 280 days assumes ovulation on day 14 of a 28-day cycle. Ovulation on day 17 of a 31-day cycle means the pregnancy is three days younger than the simple count says, so the estimate moves three days later. The page applies that correction and prints it, so a 35-day cycle produces a visibly different date rather than the same number with a different story attached.

The correction is a small fix to a rough estimate, not a replacement for a measurement. Where a scan disagrees with the rule, the scan wins, and the page says so instead of quietly preferring its own number.

Scans and IVF: when the date stops being an estimate

A dating scan measures the baby - crown-rump length in the first trimester, head circumference later - and turns that measurement into a gestational age. The due date is then 280 days from the scan date minus however far along the scan said you were. A scan at 12 weeks 3 days, taken on 1 May, gives a due date of 10 November: 193 days later.

The earlier the scan, the better it is as an anchor, because the normal range of size at a given age is narrow at 12 weeks and wide at 30. That is why a first-trimester scan is the reference most services use, and why a due date can move after a scan that disagrees with the rule.

An IVF transfer date is arithmetic rather than measurement. A day-5 blastocyst transfer happens 19 days into the 280, so the due date is 261 days later; a day-3 transfer happens on day 17, so the due date is 263 days later. The page uses those offsets exactly.

What the milestones mean, and what they do not

The table shows the conventional antenatal weeks: the dating scan window around 8 weeks, the end of the first trimester at 13 weeks 6 days, the anatomy scan around 20 weeks, viability at 24 weeks, the third trimester at 28 weeks, term at 37 weeks, the due date at 40 weeks and post-term at 42 weeks.

These are the weeks the antenatal schedule is built around, and they are conventions rather than rules about any individual pregnancy: a scan may be booked in a window of several weeks, and the point at which a particular service induces is a local policy. The page says the dates are for a conversation with a clinician, because that is what they are for.

A trimester boundary is a convention too. This page uses the usual one - 0 to 13 weeks 6 days, 14 weeks to 27 weeks 6 days, 28 weeks onwards - and it says which one it used rather than leaving the reader to guess why a date fell on one side of a line.

What this page will not tell you

It does not estimate the sex of the baby, weight at birth, or any risk, and it does not read a scan report: it takes the weeks and days you type. It does not know your medical history, your cycle regularity or whether the transfer was fresh or frozen.

It also does not adjust for a pregnancy that is already past its estimated date beyond saying so. Monitoring and induction decisions between 41 and 42 weeks are clinical, vary by service and by circumstance, and a calculator that recommended one would be giving medical advice it has no business giving.