15 Weeks Pregnant: When Did I Conceive?

Pregnancy weeks. Updated October 11, 2026. Calculated with the standard pregnancy dating method used by ACOG and the NHS.

If you are 15 weeks pregnant today, conception most likely happened about 13 weeks ago. That's because pregnancy is counted from the first day of your last period, roughly two weeks before you actually conceived.

Why the numbers don't match

Doctors and midwives count gestational age from the first day of the last menstrual period, because that date is usually known and conception usually isn't. In a 28-day cycle, ovulation and conception happen around two weeks later, which is why fetal age is about two weeks less than the number of weeks pregnant.

If your cycles are longer than 28 days, conception was likely a little more recent; with shorter cycles, a little longer ago.

What's happening at 15 weeks

Many people find the second trimester easier, with more energy and less nausea.

Working out the exact date

To get the actual calendar date of conception, use the pregnancy week calculator with the first day of your last period, or the conception date calculator if you already know your due date.

This article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.