Pregnancy Due Date Calculator
Instant results as you type.
A due date is calculated from the first day of the last period using Naegele's rule — 280 days, adjusted here if your cycle is not 28 days, since ovulation shifts with cycle length. It is worth understanding what the date is: a median, not an appointment. Fewer than one in twenty babies arrive on it, and anything from 37 to 42 weeks is full term.
How to use it
- Enter the first day of your last menstrual period.
- Enter your usual cycle length if it is not 28 days.
- Read the estimated due date, how far along you are, and the trimester.
Examples
- A last period starting 1 May 2026 with a 28-day cycle gives 5 February 2027.
- The same date with a 35-day cycle gives 12 February — a week later.
Frequently asked questions
- How is a due date calculated?
- Naegele's rule adds 280 days — forty weeks — to the first day of the last menstrual period. It assumes a 28-day cycle with ovulation on day 14, so this calculator adjusts when you enter a different cycle length. Note that dating from the last period means the first two weeks of a pregnancy predate conception.
- How accurate is an estimated due date?
- As a single date, not very — under 5 per cent of babies are born on it. As a centre point it is reliable: most births fall within two weeks either side, and 37 to 42 weeks is considered full term. Treat it as the middle of a window rather than a deadline.
- What if my cycle is not 28 days?
- Then the standard rule is off by the difference, because ovulation happens roughly 14 days before the next period rather than 14 days after the last. A 35-day cycle pushes the estimate about a week later, which this calculator applies. Very irregular cycles make period-based dating unreliable, and a scan is the better basis.
- Is a dating scan more accurate than this?
- Yes, particularly in the first trimester, where measuring the embryo dates a pregnancy to within a few days. Clinicians routinely revise a period-based date when an early scan disagrees materially. Use this calculator for orientation, and let the scan set the date that goes in your notes.