Pregnancy Due Date Calculator

Due Date Calculator

The estimated due date is calculated as 280 days — forty weeks — from the first day of the last menstrual period, or 266 days from conception. The forty-week convention embeds an assumption worth knowing about: it counts from a point roughly two weeks before conception actually occurred, because the last period is a date most people can recall while the moment of conception is not. This is why a pregnancy described as eight weeks along is typically six weeks past conception. The calculator returns the estimated due date along with the current gestational age and the trimester boundaries. Treat the date as the centre of a range rather than an appointment: only about 4% of babies arrive on it, and the great majority are born within two weeks either side.

How this is calculated

Naegele's rule, the standard method, adds 280 days to the first day of the last menstrual period. The traditional formulation is the same thing stated differently: take the first day of the last period, subtract three months, and add seven days and one year.

Worked example with a last period beginning 1 March: subtracting three months gives 1 December of the previous year, adding seven days gives 8 December, and adding a year gives 8 December of the current year as the estimated due date.

Dating from conception uses 266 days instead, since conception typically occurs about two weeks after the period begins. If you know the conception date — from IVF, for instance — that route is more accurate than the period-based one.

Why the due date is a range, not a date

Naegele's rule assumes a 28-day cycle with ovulation on day 14. Cycle length varies substantially between people and between cycles, and a longer or shorter cycle shifts the real due date accordingly — someone with a consistent 35-day cycle will typically deliver about a week later than the rule predicts.

Term is defined as 37 to 42 weeks, a five-week window, which is a fair reflection of the natural variation. Around 4% of births occur on the estimated date itself, and roughly 90% within two weeks either side of it.

A first-trimester ultrasound gives more accurate dating than the last menstrual period, because early fetal size varies little between pregnancies. Where the two disagree by more than about a week, clinicians generally revise the due date to the scan.

How the trimesters divide

The first trimester runs from week 1 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to birth. The boundaries are conventions rather than sharp biological transitions, though they correspond loosely to real shifts in risk and development.

Gestational age is always counted from the last menstrual period, which is why the count starts about two weeks before conception. This is a persistent source of confusion — it means the first two weeks of a forty-week pregnancy precede the pregnancy itself.

How to use the due date calculator

Enter the first day of your last period. The first day of bleeding, not the last. This is the date clinicians use.
Read the estimated due date. It is 280 days from the date you entered. Treat it as the centre of a five-week term window.
Check against a dating scan. A first-trimester ultrasound dates a pregnancy more accurately than the last period, particularly if your cycle is not close to 28 days.

Frequently asked questions

How accurate is a due date?

Only about 4% of babies arrive on the estimated date; most are born within two weeks either side. A first-trimester ultrasound gives the most accurate dating.

Why is pregnancy counted from the last period rather than conception?

Because the first day of the last period is a date most people can recall, while the moment of conception usually is not. It means the count starts about two weeks before conception — so eight weeks pregnant is roughly six weeks past conception.

Does an irregular cycle change the due date?

Yes. Naegele's rule assumes a 28-day cycle with ovulation on day 14. A consistently longer cycle shifts the real due date later by roughly the difference — someone on a 35-day cycle typically delivers about a week after the rule predicts. A dating scan resolves this.

When do the trimesters start and end?

First trimester: weeks 1 to 13. Second: weeks 14 to 27. Third: week 28 to birth. These are conventions rather than sharp biological boundaries, though they track loosely with shifts in risk and development.