Pregnancy Due Date Calculator

How this is worked out

Naegele's rule: the estimated due date is 280 days — forty weeks — from the first day of your last menstrual period.

That rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation is later or earlier, and the due date moves with it:

due date = LMP + 280 days + (cycle length − 28)

A 35-day cycle ovulates around day 21, so the due date is seven days later than the unadjusted rule gives. Calculators that skip this adjustment send people to a dating scan expecting a number a week out, and the scan gets treated as a surprise rather than a correction.

Gestational age is counted from the last period, not from conception — so at "eight weeks pregnant" you conceived about six weeks ago. This is universal medical convention and it is the source of a great deal of confusion, because it means the clock starts about two weeks before there is anything to count.

A worked example

The due date is the middle of a range, not a deadline

About four per cent of babies arrive on their estimated due date. Full term spans 39 to 40 weeks; anything from 37 weeks is early term and 42 is post-term. The single date is the centre of a distribution that is several weeks wide, and treating it as an appointment is the source of a great deal of unnecessary anxiety in the last fortnight.

First pregnancies run slightly longer on average than subsequent ones. Both sit comfortably inside the normal range, and neither is a problem.

The dates that carry real clinical weight are the boundaries: 37 weeks, when a baby is considered early term rather than premature; 39 weeks, the start of full term; and 42 weeks, when most care teams will discuss induction. The milestone table on this page gives all of them.

Why the cycle-length adjustment matters

Naegele's rule was published in the early nineteenth century and assumes every cycle is 28 days with ovulation exactly halfway. Real cycles range from about 21 to 35 days, and the variation is almost entirely in the first half — the follicular phase — while the luteal phase after ovulation is stable at around 14 days.

So someone with a 35-day cycle ovulates around day 21 rather than day 14. Conception happened a week later than the rule assumes, and the due date is a week later too. Someone with a 24-day cycle is four days earlier.

This is a large enough difference to matter at both ends of a pregnancy — for when a dating scan is offered, and for when a post-term conversation begins. It costs one extra field to get right, and this calculator asks for it.

A dating scan beats any formula

An ultrasound in the first trimester measures the embryo directly and is accurate to within about five days. It does not depend on remembering a date correctly or on your cycle behaving as expected, and if it disagrees with the calculated date by more than about a week, the scan date is the one your care team will use.

Accuracy falls as pregnancy progresses, because babies grow at increasingly different rates. A scan at 12 weeks is a good estimate; the same measurement at 30 weeks is not, which is why dating is done early and then left alone.

Treat this calculator as a way to know roughly where you are before the first appointment, and as a reference for the milestones afterwards. It is not a substitute for the scan.

Why the count starts before conception

Gestational age is measured from the first day of the last period, which is roughly two weeks before conception. It means that at the moment of conception you are already "two weeks pregnant", and that at the point most people take a test — around four to five weeks — the embryo has existed for two or three.

The convention exists because the last period is a date people can usually identify, and the moment of conception is not. It is not a mistake, but it does mean every week number you hear is about two weeks ahead of the embryo's actual age.

Fertility clinics use a different convention for IVF, dating from the transfer, which is why an IVF due date is calculated differently and should come from the clinic rather than from a calculator like this one.

Assumptions and sources

Due date formula

Naegele's rule: last menstrual period + 280 days, adjusted by (cycle length − 28).

Term definitions

Early term from 37 weeks, full term 39–40 weeks, late term 41 weeks, post-term from 42 weeks. checked 2026-08

Proportion born on the due date

Approximately 4%. The due date is the centre of a distribution several weeks wide.

Dating scan accuracy

First-trimester ultrasound dates a pregnancy to within about five days, and takes precedence over a calculated date where the two disagree materially.