How your due date is calculated

Your due date is estimated as 40 weeks, or 280 days, from the first day of your last menstrual period. This is Naegele's rule, the standard the NHS and most clinicians use. From a known conception date the count is 266 days, and for IVF it is the transfer date plus 266 days minus the embryo's age. Only about 1 in 20 babies arrive on the exact due date, and your midwife may revise it after your dating scan.

Informational only, not medical advice. Reviewed by our editorial team against NHS and gov.uk guidance. Speak to your midwife, GP or NHS 111 about any pregnancy concern.

The 280-day rule

A full-term pregnancy is dated as about 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). Adding 280 days to the LMP gives the estimated due date. The NHS dates pregnancy this way, counting from the LMP rather than from conception, because the LMP is a date most people can identify while the exact day of conception usually is not known.

From conception or IVF instead

If you know the conception date, the count is 266 days (38 weeks), because ovulation and conception happen about 14 days after the LMP. For IVF the embryo's age is already known, so the due date is the transfer date plus 266 days minus the embryo age, which is commonly 3 or 5 days. Whichever method you use, the number of weeks you are now is still counted from the effective LMP, as the NHS does.

Why a scan can move your date

A due date from the LMP assumes a regular 28-day cycle with ovulation around day 14. Cycles vary, so the dating scan, usually between 8 and 14 weeks, measures the baby and can adjust the date. The scan date then becomes your official due date. This is normal and does not mean anything is wrong.

Work out your own due date

The BabyData due date calculator applies these exact rules from your LMP, conception date or IVF transfer date, and also tells you how many weeks pregnant you are now. To check your progress later, use the pregnancy weeks tool.

Sources

• NHS: working out your due date and how pregnancy is dated.
• Naegele's rule (LMP plus 280 days), the standard clinical dating method.