Due Date Calculator

How this is calculated

due date = LMP + 280 days (Naegele's rule, 40 weeks)

Example: LMP 1 January → due 8 October. Pregnancy weeks count from the LMP too, which is why "6 weeks pregnant" is possible four weeks after conception. Only ~5% of babies arrive on the date itself; the two weeks either side are all normal. A first-trimester scan may refine the estimate — when scan and dates disagree, clinicians go with the scan. Informational only — your antenatal care sets the clinical dates.

What Naegele's rule assumes

The standard calculation adds 280 days to the first day of the last menstrual period — equivalently, one year minus three months plus seven days. It carries two assumptions that a great many pregnancies do not meet: a 28-day cycle, and ovulation on day 14. Cycles of 21 to 35 days are entirely normal, and ovulation timing varies both between people and month to month for the same person. A consistently 35-day cycle shifts the true date about a week later than the rule predicts.

Franz Naegele formalised the rule in 1812, and it has survived because it is simple, needs one date, and is right on average. Being right on average is exactly the limitation: it describes the middle of a distribution, not your pregnancy.

Why an early scan supersedes this page

A first-trimester ultrasound measures the embryo directly — crown-rump length — and early growth is consistent enough between pregnancies that this dates a pregnancy to within about five days. That is considerably better than a calculation resting on a remembered date and an assumed cycle, which is why obstetric guidelines say the scan date takes precedence when the two differ by more than about a week. After roughly 14 weeks, growth variation widens and ultrasound dating becomes progressively less precise, which is why an early scan matters and a later one dates less well.

For IVF pregnancies neither applies: the date of transfer is known exactly, so the due date is calculated from it and is the most accurate of all.

Almost nobody delivers on the date

Fewer than one birth in twenty happens on the estimated due date. Roughly half occur within a week either side, and the great majority within two. First pregnancies tend to run slightly longer than subsequent ones. Term itself is now defined as a range rather than a point: early term at 37–38 weeks, full term at 39–40, late term at 41, and post-term beyond 42, a change made specifically because treating 40 weeks as a deadline led to interventions that did not improve outcomes.

The useful mental model is a due month rather than a due date. Plan for the fortnight either side: that is when leave should start being flexible, when the bag should be packed, and when travel should stop.

What the milestone dates on this page mean

The trimester boundaries are conventions rather than biological events, but they line up with real changes. The end of the first trimester at around 13 weeks marks the point where miscarriage risk drops substantially. Viability is usually discussed from around 24 weeks, though outcomes improve week by week well beyond that, and the specific threshold varies by country and by the neonatal care available. The 36-week mark is where most airlines stop accepting pregnant passengers, and where many care plans expect you to stay close to home.

This calculator is a planning aid. It cannot know your cycle length, your scan measurements or your history, and it is not a substitute for antenatal care. Every date it shows is an estimate with a fortnight of legitimate slack around it.