How your due date is calculated
Most due dates are worked out with Naegele’s rule: take the first day of your last menstrual period (LMP) and add 280 days, or 40 weeks. This assumes a 28-day cycle with ovulation around day 14. If your cycles are longer or shorter, ovulation is likely to happen later or earlier, so this calculator shifts the date by the difference. A 32-day cycle moves the due date four days later, and a 25-day cycle moves it three days earlier.
For example, if your last period started on January 1 and your cycle is 28 days, your estimated due date is October 8. Conception most likely happened around January 15, two weeks after the LMP.
Weeks pregnant and trimesters
Pregnancy is counted from the LMP, not from conception, so at the moment of conception you are already considered about two weeks pregnant. The calculator shows how many weeks and days pregnant you are on any date you choose. The first trimester runs 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. The milestones table lists useful dates such as the usual window for the mid-pregnancy anatomy scan (18 to 22 weeks) and when you reach full term at 39 weeks.
How accurate is a due date?
A due date is an estimate. Only about 4 to 5% of babies are born on the exact day, and most arrive within two weeks either side. Babies born between 37 and 42 weeks are considered on time, with 39 weeks 0 days to 40 weeks 6 days classed as full term. First pregnancies tend to run a little longer on average.
An early ultrasound, ideally between 8 and 13 weeks, measures the baby’s crown-to-rump length and is the most accurate way to date a pregnancy. If the scan differs from the LMP date by more than about a week, your provider will usually go by the scan. Dates from IVF are even more precise, because the embryo transfer date is known.
When this calculator may not fit
LMP dating is less reliable if your cycles are irregular, if you recently stopped hormonal birth control, if you are breastfeeding, or if you are unsure of the date your last period started. In those cases treat the result as a rough guide until you have had a dating scan. If you have any bleeding, severe pain or other concerns at any stage of pregnancy, contact your midwife, doctor or local emergency services rather than relying on an online tool.
Frequently asked questions
How do I calculate my due date?
Add 280 days (40 weeks) to the first day of your last menstrual period, adjusting by the number of days your cycle is longer or shorter than 28.
Can my due date change?
Yes. Your provider may update it after an early ultrasound if the baby’s measurements suggest a different date.
How many babies are born on their due date?
Only about 1 in 20. Most babies are born within two weeks before or after the estimated date.
When does the second trimester start?
At 14 weeks 0 days, counted from the first day of your last period.