How the due date is calculated
Pregnancy is counted from the first day of your last menstrual period (LMP), about two weeks before conception. A typical pregnancy lasts 280 days (40 weeks) from that date. This is Naegele's rule: add 280 days to the LMP, adjusted for cycles longer or shorter than 28 days.
- From conception: due date = conception + 266 days.
- After IVF: due date = transfer date + 266 days − the embryo's age (3 or 5 days).
- From an ultrasound: an early scan (before about 14 weeks) is the most accurate way to date a pregnancy; your provider may change your due date based on it.
Trimesters and milestones
The first trimester runs to 13 weeks 6 days, the second to 27 weeks 6 days and the third until birth. A baby born from 37 weeks is considered term; 39–40 weeks is “full term”.
Only about 4–5% of babies arrive exactly on their due date; most are born within two weeks either side of it.
Frequently asked questions
How accurate is a due date?
It's an estimate. Most babies are born between 38 and 42 weeks, and only about 1 in 20 on the exact date. An early ultrasound gives the most reliable dating.
How many weeks pregnant am I?
Count the weeks since the first day of your last period. The calculator shows it as weeks and days, for example 12 weeks 3 days.
When does the second trimester start?
At 14 weeks 0 days, counted from the first day of your last period.
Can I calculate my due date from conception?
Yes: add 266 days (38 weeks) to the date of conception, or choose “Conception date” above.
This calculator gives an estimate for information only. Your doctor or midwife will confirm your due date, usually with an ultrasound.