How due dates are calculated
Pregnancy is dated from the first day of your last menstrual period (LMP), counting 40 weeks (280 days) — even though conception happens about two weeks later. That's the standard Naegele's rule that doctors and pregnancy apps use.
What it means
- Due date is an estimate — only about 4–5% of babies arrive exactly on their due date; most come within two weeks either side.
- Dating scan (around 8–14 weeks) is more accurate than LMP-based calculation and may adjust your date.
- Trimesters: first trimester ends ~13 weeks, second ends ~27 weeks, third runs to birth.
- If your cycle is irregular, the LMP method is less reliable — conception-date and ultrasound dating help.
This tool is informational, not medical advice — always follow your healthcare provider's guidance.