When Did Conception Occur?
Calculate the probable conception date from your due date or last menstrual period.
When Does Conception Happen?
Conception (fertilization) typically occurs about 2 weeks after the first day of your last menstrual period — on or near the day of ovulation. However, sperm can survive up to 5 days, so intercourse up to 5 days before ovulation can result in conception. The fertilized egg takes another 6-10 days to implant in the uterus.
Why Due Dates Use LMP, Not Conception
Medical due dates are calculated from the Last Menstrual Period (LMP), not conception, because most women know their LMP date but not their exact conception date. Gestational age (40 weeks from LMP) is about 2 weeks longer than fetal age (38 weeks from conception). This is why you are already "2 weeks pregnant" on your conception date.
First trimester ultrasound (6-9 weeks) is the most accurate method for determining gestational age, accurate to within 3-5 days. After the first trimester, accuracy drops to 1-2 weeks. If your ultrasound date and LMP date disagree by more than 7 days, the ultrasound date is used for the due date.
