The date of conception is generally estimated as the day when fertilization occurs, which can be roughly around 11-21 days after the first day of the last menstrual period (LMP) of a woman with a regular cycle. It is difficult to pinpoint the exact day of conception because it depends on the timing of ovulation and fertilization. Conception technically requires both fertilization of the egg by sperm and implantation of the fertilized egg into the uterus. To estimate when conception likely occurred:
- If you know the first day of the last menstrual period, conception usually occurs about 11-21 days after that.
- If you know your due date, conception is estimated by subtracting 266 days (38 weeks) from that date.
- Ultrasound measurements in early pregnancy give a more accurate conception window.
Pregnancy calculators use these methods to estimate the conception date, but exact precision is challenging because sperm can survive in the female reproductive tract for up to 3-5 days, and ovulation timing varies.
