Pregnancy Due Date Calculator

Enter the first day of your last period — get your estimated due date, current week of pregnancy and trimester. Everything stays on your device.

How the estimate works

The standard method — Naegele's rule — adds 280 days (40 weeks) to the first day of your last menstrual period. If your cycle isn't 28 days, the estimate shifts by the difference: a 32-day cycle moves it 4 days later, a 26-day cycle 2 days earlier. Pregnancy weeks are counted from the LMP too, which is why "week 6" begins only about four weeks after conception.

Remember it's an estimate: only about 1 in 20 babies arrives on the exact due date, and anywhere from 37 to 42 weeks is considered normal timing. A first-trimester ultrasound gives the most accurate dating — if it differs from the LMP estimate, clinicians normally trust the ultrasound.

The three trimesters

Pregnancy is divided into three trimesters of roughly 13 weeks each. The first trimester (weeks 1–12) is when major development and early symptoms occur; the second (weeks 13–27) is often the most comfortable; the third (weeks 28–40) is the final growth phase before birth. The calculator shows which trimester you are currently in.

Due date vs conception date

Because ovulation happens about two weeks into the cycle, pregnancy is dated from the first day of your last period, not from conception — so "week 6" is only about four weeks after conception. This convention (Naegele's rule) is what doctors and scans use, which is why the estimate can differ from when you think conception occurred.

Why only about 4% of babies arrive on the due date

The due date is the midpoint of a range, not an appointment. Naegele's rule assumes a 28-day cycle with ovulation on day 14, and neither is universal — which is why most births land within a fortnight either side rather than on the day itself.

Roughly speaking, about 4% arrive on the estimated date, half within a week of it, and around 90% between 37 and 42 weeks. Only about 10% of pregnancies reach 42 weeks, at which point most clinicians discuss induction. First pregnancies also tend to run a few days longer than subsequent ones.

If your cycle is consistently longer or shorter than 28 days, adjust: someone with a 32-day cycle typically ovulates around day 18, so their true due date is about four days later than the standard formula suggests.

How dating scans revise the estimate

An early ultrasound is more accurate than the last-period calculation, and clinicians will change the due date if the two disagree meaningfully. Measuring crown-rump length between 8 and 13 weeks dates a pregnancy to within about five days — the most reliable window there is.

Accuracy falls as pregnancy progresses, because babies grow at increasingly different rates: a second-trimester scan is accurate to roughly 1–2 weeks, and a third-trimester scan to 3 weeks or more. This is why a late scan showing a "big baby" does not reliably move the due date, and why the earliest scan usually stands as the official one.

Counting weeks and knowing the milestones

Pregnancy is counted from the first day of your last period, not from conception — so you are considered two weeks pregnant at the moment of conception. It sounds odd, but it exists because the last period is a date most people can identify reliably, while conception usually is not.

Milestones worth having in mind: week 12 ends the first trimester and miscarriage risk drops considerably; weeks 18–22 is the anomaly scan; week 24 is generally considered the threshold of viability; week 28 starts the third trimester; week 37 counts as full term; week 40 is the due date; and week 42 is post-term.

Frequently asked questions