How it works
Due dates are usually estimated with Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Pregnancy weeks are counted from the LMP, so conception typically happens around week 2.
Only a small share of babies arrive on their exact due date — birth between weeks 38 and 42 is considered normal. An ultrasound can give a more precise estimate.
For information only. Always follow your doctor's or midwife's guidance.