Pregnancy Due Date Calculator

Naegele's rule

The standard calculation adds one year, subtracts three months, and adds seven days to the first day of the last menstrual period. That is Naegele's rule, and it gives 280 days from the last period.

It assumes a 28 day cycle with ovulation on day 14, which is why the pregnancy is counted from two weeks before conception occurred.

For cycles longer or shorter than 28 days the rule needs adjusting by the difference, since the follicular phase is the variable part. A 35 day cycle shifts the due date a week later than the unadjusted calculation gives.

Ultrasound dating

An early ultrasound dates a pregnancy more accurately than menstrual dates. Between roughly 8 and 14 weeks, crown-rump length predicts gestational age within about five days.

Where the scan and the menstrual dates disagree by more than a few days, the scan date is used. UK practice is to date by the dating scan, offered between 11 and 14 weeks, and this can move a due date by a week or more.

Later scans are less accurate for dating because fetal growth becomes more variable between individuals. A scan at 30 weeks estimates size well and gestational age poorly.

How little the date predicts

Around 4% of babies arrive on their due date. Term spans 37 to 42 weeks, which is a five week window, and most births fall within it.

Research measuring pregnancies from precisely dated conception found natural variation of around 37 days between the shortest and longest, in pregnancies without complications.

Which is why the phrase estimated due date matters. It is the centre of a distribution, and treating it as an appointment causes a great deal of unnecessary anxiety in the days after it passes.

What happens after 40 weeks

Risks including stillbirth rise gradually after 41 weeks, which is why induction is offered rather than waiting indefinitely.

UK guidance offers induction between 41 and 42 weeks, with membrane sweeps offered from 40 weeks in a first pregnancy. Practice varies between countries and between units.

The decision involves the individual pregnancy, the mother's preferences, and any complicating factors, and it is made with the midwifery and obstetric team. A calculated date informs that conversation and does not determine it.

The trimesters and what happens in each

The first trimester runs to 13 weeks and covers organ formation, which is why it carries the highest risk of miscarriage and why advice about medication and alcohol is most emphatic then.

The second, 14 to 27 weeks, is when most women feel best, when movement is first felt, and when the anomaly scan is offered around 18 to 21 weeks.

The third, 28 weeks to birth, is growth and maturation, with lung development continuing to the end. Viability improves rapidly across the early part of it, which is why gestational age matters so much in decisions about early delivery.

One thing worth knowing about how the date is used in practice: it drives the whole schedule of care rather than only the anticipated birth. Screening tests have windows that work only within specific gestational ages, the anomaly scan is offered in a defined range, and decisions about early delivery depend on gestational age to within days. That is the real reason dating accuracy matters clinically, and it is why a scan date supersedes a calculated one even where the calculated one was based on confidently remembered dates.