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Pregnancy Due Date Calculator. And where you are now.

One date in — the first day of your last period — and out comes the estimated due date, the week you are in, and how much of it is left.

Live calculation

When is it due?

Based on a typical 28-day cycle with ovulation around day 14. A notably longer or shorter cycle shifts the estimate by roughly the same amount.
An estimate, not a prediction — about 1 birth in 20 lands on the date, and 37 to 42 weeks is all considered full term. A dating scan beats this calculation.
Estimated due date —
Current week
—
Trimester
—
Days to go
—

Your figures are kept on this device only.

How it works

This calculator uses Naegele's rule, the standard estimate obstetricians start from: add 280 days (40 weeks) to the first day of your last period. It is a starting estimate based on averages, not a prediction — only around 1 in 20 births land on the exact estimated date, and a full-term birth anywhere from 37 to 42 weeks is considered normal.

estimated due date = last period date + 280 days
current week = (today − last period date) ÷ 7, rounded down
conception is estimated around day 14 of the cycle

Pregnancy is dated from the last period rather than from conception because the last period is a known date, while ovulation and conception usually are not — which is why a pregnancy is described as 40 weeks even though conception happens roughly two weeks after the cycle starts. An ultrasound dating scan, once available, is generally more accurate than this calculation and should take precedence over it.

Worked example

A last period that started 10 weeks and 3 days ago.

Step by step
  1. Days since the last period: 10 × 7 + 3 = 73 days.
  2. Current week: 73 ÷ 7 = 10 weeks, 3 days — still the first trimester.
  3. Estimated due date: last period date + 280 days.
  4. Days to go: 280 − 73 = 207 days from today.

At 10 weeks 3 days, the estimate lands in the first trimester, with about 207 days — roughly 29 weeks — left to go.

Common questions

How accurate is this estimate?

It is a reasonable starting point when that is all you have, but cycle length varies between people and month to month, and Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14. A first-trimester dating ultrasound is the more accurate reference point once one is available — this calculator is for orientation beforehand, not a diagnosis.

Why are trimesters counted the way they are?

There is no single official cutoff, but a common convention is weeks 1–13 for the first trimester, 14–27 for the second, and 28 onward for the third — that is what this calculator uses. Some care providers draw the lines a week or two differently, which is normal.

What if my cycle isn't 28 days?

The estimate shifts by roughly the same amount your cycle differs from 28 days. A 36-day cycle, for example, tends to mean ovulation and conception happened later than day 14, so the real due date is often somewhat later than this calculator suggests — a scan can confirm this more precisely than adjusting the date by hand.