Conception Date Calculator

Conception — the moment of fertilization — happens around ovulation, about 14 days after the start of your last period. Enter the date of your last period to estimate when you likely conceived.

How the date is estimated

The estimate uses conception ≈ last period + 14 days, the same assumption behind Naegele’s rule, which sets a due date by adding 280 days to the first day of the last period. As StatPearls spells out, that rule “assumes that the patient has a normal 28-day menstrual cycle and ovulates on day 14 of that cycle.” In longer or shorter cycles, ovulation shifts and the conception date follows.

Treat the result as a midpoint, not a date. In a pooled analysis of 3,324 cycles from 581 U.S. women, ovulation fell on cycle day 14 in only 8.7% of cycles — the day-14 rule describes the average cycle, not most cycles. For the same reason, a first-trimester ultrasound (accurate to about 5 to 7 days) overrides the calendar when the two disagree by more than a few days.

Worked example

Last period started March 1:

• Estimated conception: around March 15
• Gestational age today: about 2 weeks more than the time since conception

That two-week offset isn’t a rounding quirk. Gestational age is counted from the last period, while conceptional age is counted from fertilization — so a pregnancy that is “8 weeks pregnant” contains an embryo about 6 weeks old.

Why it’s not the day of intercourse

Sperm can live 3 to 5 days in the reproductive tract, per the Office on Women’s Health, while the egg survives only 12 to 24 hours after it is released. So intercourse a few days before ovulation can result in fertilization on the day the egg appears. That’s why the conception date sits closer to ovulation than to intercourse itself.

If you tracked ovulation, you’ll have a better estimate — see the ovulation symptoms and the fertile window calculator.

This calculator is informational and gives an estimate. Pregnancy dating is confirmed through prenatal care.