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Pregnancy Calculator

Tell the calculator what you know — the first day of your last period, the day of conception, or a due date someone gave you — and it works out the rest: the date, the week you are in now, and how much is left.

The due date is an estimate, not an appointment: about one baby in twenty arrives on it. The whole calculation rests on the assumption that ovulation happened on day 14 of the cycle — that is why the cycle length is adjustable here. An ultrasound dates a pregnancy more accurately than any arithmetic, and a doctor’s date should always win over this one.

How to use

  1. Choose what you are counting from.
  2. Pick the date, and correct the cycle length if yours is not 28 days.
  3. Read the due date, the current week and the key dates below.

Good to know

How the date is worked out

By Naegele’s rule: 280 days — forty weeks — from the first day of the last period. It is the only calculation possible without an ultrasound, and it rests on one assumption: that ovulation happened on day 14 of the cycle. That is true for many people and not for everyone, which is why the cycle length is adjustable here. At a 32-day cycle ovulation falls around day 18, and every date on this page moves four days later.

Why the date is an estimate

About one baby in twenty arrives on the estimated day; most come within the two weeks either side of it, which is exactly why 37 to 42 weeks counts as term. An ultrasound in the first trimester dates a pregnancy far more accurately than any arithmetic on dates, because it measures the baby rather than the calendar. If a doctor has given you a date, that date wins over this one — the calculator here is for the weeks in between.

Frequently asked questions

How accurate is this?

The arithmetic is exact; the assumption underneath it is not. Everything depends on ovulation having been on day 14 of your cycle. If your cycle is regular and you know its length, the answer is close. If it is irregular, treat the result as a rough bearing and rely on the ultrasound date.

My cycle is not 28 days.

Change the number in the field. Ovulation happens about 14 days before the next period, not 14 days after the last one, so a longer cycle pushes conception — and every date here — later by the difference.

It says the numbers do not add up.

Two things can cause that: a date still in the future, or a date more than 48 weeks back — usually a typo in the year. Rather than show you a negative term, the calculator says which of the two it is.

Is what I type sent anywhere?

No. The whole calculation runs in your browser, on your device. Nothing is stored and nothing leaves — this is one page where that matters more than usual.

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