How is a due date calculated?

By adding 280 days to the first day of your last period, adjusted for cycle length. Pregnancy is dated from that day rather than from conception.

How accurate is a due date?

Only about 4 per cent of babies are born on it, though roughly 80 per cent arrive within two weeks either side. Treat it as a midpoint.

What if my cycle is not 28 days?

The estimate shifts. Ovulation is roughly 14 days before your next period, so a 32-day cycle moves the due date about four days later.

Which is more accurate, dates or a scan?

A dating scan, especially for irregular cycles. If scan dates differ from your calculated date, maternity teams normally use the scan.

Due Date Calculator - Estimate Your Delivery Date

Pregnancy Due Date Calculator

A due date is an estimate calculated from your last period and adjusted for cycle length. Very few babies arrive on it, which is worth knowing from the start.

Formula

Due date = first day of the last period + 280 days

Naegele’s rule, which assumes a 28-day cycle ovulating on day 14
A longer or shorter cycle shifts the date by the same difference

Last period 1 January: due date around 8 October

How the Date Is Calculated

The standard method adds 280 days (40 weeks) to the first day of your last menstrual period.

That assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the estimate shifts: on a 32-day cycle ovulation falls around day 18, pushing the due date about four days later.

Pregnancy is dated from the last period rather than conception, which means you are counted as two weeks pregnant before conception occurs.

The Trimesters

First trimester: weeks 1–13
Second trimester: weeks 14–27
Third trimester: weeks 28–40

Full term is 37 to 42 weeks. Anything before 37 weeks is considered preterm.

How Accurate Is It?

Only about 4% of babies arrive on their due date, and roughly 80% are born within two weeks either side. It is a midpoint, not an appointment.

A dating scan, usually offered between 8 and 14 weeks in the UK, measures the baby directly and is more accurate than the calendar method, particularly for irregular cycles. If the scan date differs, your maternity team will normally use theirs. This calculator is an estimate and no substitute for antenatal care.

Quick Reference

The pregnancy timeline from the last menstrual period.
Milestone	Weeks	Days from LMP
Conception (typical)	2	14
First trimester ends	13	91
Second trimester ends	27	189
Full term begins	37	259
Due date	40	280
Post-term	42	294

Frequently Asked Questions

How is a due date calculated?: By adding 280 days to the first day of your last period, adjusted for cycle length. Pregnancy is dated from that day rather than from conception.
How accurate is a due date?: Only about 4 per cent of babies are born on it, though roughly 80 per cent arrive within two weeks either side. Treat it as a midpoint.
What if my cycle is not 28 days?: The estimate shifts. Ovulation is roughly 14 days before your next period, so a 32-day cycle moves the due date about four days later.
Which is more accurate, dates or a scan?: A dating scan, especially for irregular cycles. If scan dates differ from your calculated date, maternity teams normally use the scan.