How your due date is worked out
Doctors count pregnancy from the first day of your last menstrual period (LMP), not from conception. Naegele's rule adds 280 days to that date. For a cycle that is not 28 days, the date shifts by the difference, because ovulation happens later or earlier. If you know when you conceived, for example after IVF or one known date, the calculator adds 266 days instead.
What you see in the result
You get the estimated due date, your gestational age today (weeks and days), your trimester and the dates when common milestones fall: the 20-week scan window, the glucose test window, full term and the end of the 40th week. Use the add or subtract days calculator or the week number calculator for other date questions, and the BMI calculator or water intake calculator for general health numbers.
This is general information, not medical advice. Only a doctor or midwife can confirm your due date.
Frequently asked questions
How is the due date calculated?
The usual method is Naegele's rule: take the first day of your last menstrual period and add 280 days (40 weeks). If your cycle is longer or shorter than 28 days, the date is moved by the difference. If you know the conception date, the due date is 266 days (38 weeks) after it.
How accurate is an estimated due date?
Only about 4–5% of babies arrive on the exact due date. Most are born between 37 and 42 weeks. A dating ultrasound in the first trimester is more accurate than counting from your period, and your doctor or midwife will confirm your date.
What are the three trimesters?
The first trimester runs from week 1 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Weeks are counted from the first day of your last period, so you are counted as 'pregnant' about two weeks before conception.
What does full term mean?
A pregnancy is early term from 37 weeks 0 days to 38 weeks 6 days, full term from 39 weeks 0 days to 40 weeks 6 days, and late term from 41 weeks 0 days to 41 weeks 6 days. Born before 37 weeks is preterm.
Does this replace a doctor's advice?
No. This tool gives a general estimate only. Always follow the advice of your doctor or midwife about scans, tests and your own due date.