Conception Date Calculator

How the Conception Date Calculator works

How the conception date calculator works

A pregnancy is dated as 280 days from the first day of the last menstrual period (LMP), but conception usually happens about two weeks later, at ovulation. That leaves about 266 days, or 38 weeks, from conception to birth. The formula is conception date = due date − 266 days. From a last period it is LMP + (cycle length − 14) days, because ovulation happens about 14 days before the next period. Sperm can survive up to five days, so intercourse up to five days before ovulation can lead to pregnancy.

First example. With a due date of 10 April 2027, conception most likely happened on 18 July 2026, and the fertile window ran from 13 to 19 July.

Second example. A last period starting on 1 September 2026 with a 32-day cycle points to conception on about 19 September 2026 and a due date of 12 June 2027.

How many days after sex does conception happen? Anywhere from the same day to about five days later, because sperm wait in the fallopian tubes until an egg is released.

Only about 4% of babies arrive on their due date, so working back from a birth date gives a rough answer. An early ultrasound dating scan is the most accurate guide.

Conception date by due date (266 days before the due date)

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