Why Date Pregnancy From Your Last Menstrual Period?

Added: 2026-06-03

[00:00:00]As a prenatal nurse, one of the questions that I'm asked most often is why we date a baby's age based on the first day of the woman's last period.

[00:00:09]I'm Lauren and I'm a nurse at the Pregnancy Network and often when I'm doing a woman's first ultrasound, I'll measure the baby and say, "Oh, the baby is 7 weeks and 3 days." And the woman will say, "Well, that can't be possible.

[00:00:21]I didn't have sex 7 weeks ago. I had sex 5 weeks ago."

[00:00:25]And that lines up because even though the baby was conceived 5 weeks ago, her period was probably 7 weeks ago. And the reason that we use the first day of the last period to age or date the pregnancy is because we know for sure what day that was.

[00:00:41]Most women are having sex multiple times during the month and so nailing down an exact conception date can be difficult and it can be an estimation at best.

[00:00:52]If a woman only has sex one time during the month, then we know the exact date of conception, but still we're going to use the first day of the last period because we know that date for sure. So, if we say that you are 38 weeks along, that means the baby has been in your uterus for about 36 weeks, but it's been 38 weeks since the first day of your last period and that's what we use to determine the age of the baby.