CalcKitHow to Use the Pregnancy Due Date Calculator
Doctors estimate a due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is known as Naegele's rule, and it's the same method used at most first prenatal appointments — because the exact day of conception is rarely known, while the LMP date usually is.
To use this calculator, enter the first day of your last period and press Calculate Due Date. You'll get your estimated due date, how far along you are right now (in weeks and days), your current trimester, the approximate conception date and how many days remain.
Pregnancy is divided into three trimesters: weeks 1–13 (first), 14–27 (second) and 28–40 (third). Interestingly, the "40 weeks" count starts before conception — ovulation and fertilisation typically happen around week 2, so at "4 weeks pregnant" the embryo is really about 2 weeks old.
Remember this is an estimate: only about 1 in 20 babies arrives exactly on the due date, and most arrive within two weeks on either side. The calculation also assumes a regular 28-day cycle; if your cycles are much longer or irregular, an early ultrasound gives a more accurate date. Always confirm with your healthcare provider, and pair this with tracking your baby's age after birth.