How the due date is estimated
The standard method is Naegele's rule: pregnancy is counted as 280 days (40 weeks) from the first day of your last period. Because that rule assumes a 28-day cycle with ovulation on day 14, this calculator adjusts the due date when you enter a different average cycle length — a longer cycle pushes the date later, a shorter one earlier. If you know your conception date instead, it counts 266 days (38 weeks) from that point.
Understanding gestational age and trimesters
Gestational age is measured from your last period, not from conception, so at the moment of conception you are already considered about two weeks pregnant. The tool reports how far along you are today in completed weeks and days and places you in the first (weeks 1–13), second (14–27), or third (28–40+) trimester. It also marks when full term begins at 37 weeks, since babies born from then on are considered full term.
Why the date is an estimate
Only about 4–5% of babies arrive on their exact due date; most births happen in the two weeks either side. Cycle length varies, ovulation timing varies, and early-pregnancy dating by ultrasound often adjusts the estimate. Treat the due date as the center of a likely window rather than a fixed appointment, and rely on your healthcare provider's dating, which uses ultrasound measurements for greater accuracy.
A worked example
If the first day of your last period was 3 May 2026 with a typical 28-day cycle, your estimated due date is about 7 February 2027, conception is estimated around 17 May 2026, and on 12 July 2026 you would be roughly 9 to 10 weeks along in the first trimester. Lengthen the cycle to 32 days and the due date shifts about four days later; switch to conception mode and enter a known date to bypass cycle-length assumptions entirely.
Health disclaimer
This calculator provides general guidance only and is not medical advice. It does not replace prenatal care. Always confirm your dating and discuss any concerns with a qualified healthcare provider, who will verify the due date by ultrasound and monitor the pregnancy appropriately.