Conception Calculator

The calculator subtracts exactly 266 days (38 weeks) from your due date, the reverse of Naegele's rule. That figure assumes fertilization happened 266 days before delivery. The result is an estimate because the due date itself carries uncertainty of plus or minus a week or two, and only about 4% of babies arrive on the exact due date.

A 28-day cycle puts ovulation at day 14 from the last menstrual period (LMP), which is the Naegele's rule baseline. If you enter a different cycle length, the calculator shifts the conception date to LMP plus (your cycle length minus 14) days. A 32-day cycle, for example, moves the estimate 4 days later. Valid cycle lengths are 20 to 45 days.

Reasonably accurate for people with regular cycles, but actual conception can differ by several days. Sperm survive up to 5 days inside the reproductive tract, so the calculator shows a fertile window running from 5 days before the estimated conception date through the day after. Irregular cycles widen that uncertainty further.

A first-trimester scan measures the embryo's crown-rump length directly, independent of cycle length or LMP recall. It is typically accurate within 5 to 7 days when done before 13 weeks, and accuracy drops after that. In ultrasound mode, the calculator sets conception to today minus your reported gestational age in days, then adds 14 days to convert gestational age into days since conception.

No. Every method, whether due date, LMP, or ultrasound, produces an estimate with a margin of error measured in days. For paternity questions, legal matters, or clinical decisions, consult your doctor or midwife rather than relying on any calculator result.