How Pregnancy Due Dates Are Calculated

A due date is an estimate of when a baby is likely to arrive. It is worked out with a standard rule, and then often confirmed or adjusted by a scan.

The standard rule

A pregnancy is counted as 40 weeks, or 280 days, from the first day of the last menstrual period. Pregnancy weeks are counted from that date, which is about two weeks before conception happens, so you are already two weeks pregnant at the moment of conception by this way of counting.

Cycle length matters

The rule assumes a 28 day cycle. If your cycles are longer, ovulation comes later, so the due date moves later by the same number of days. A 32 day cycle adds four days, and a 24 day cycle takes four away. If you know the conception date, add 266 days to it instead. The Due Date Calculator takes either date and your usual cycle length.

Weeks and trimesters

• First trimester: the start of pregnancy to the end of week 13.
• Second trimester: weeks 14 to 27.
• Third trimester: week 28 to the birth.
• A baby is considered term from week 37, and many doctors call weeks 39 to 40 full term.

How accurate is the date?

Only about 5 in 100 babies arrive on the due date. Most come within a couple of weeks either side. An early ultrasound measures the baby and can give a more accurate date than the calendar, especially if your cycles are irregular, and your midwife or doctor will tell you which date to use.

Other date sums

To count the days or weeks between two dates, use the Date Calculator, and to find an exact age in years, months and days, use the Age Calculator. This is general information, not medical advice.