Pregnancy weeks calculator

Gestational age is counted from the first day of your last period, not from conception. That is why a pregnancy is usually already four or five weeks along when it is confirmed.

The “weeks + days” format is the one used in obstetric care.

Days since the last period are counted and divided by seven. The estimated due date falls at 280 days, that is 40 weeks.

The two weeks you were not pregnant

Obstetric convention counts from the first day of the last menstrual period, not from conception. Since ovulation arrives about two weeks later, when you are told you are six weeks along the embryo is about four. It is not an error: it is the system all of obstetrics uses because the date of a period is remembered and the date of conception almost never is.

The trimesters and what happens in each

The first trimester runs to week 13, and organs form during it. The second, weeks 14 to 27, is when the anomaly scan is done and first movements are usually felt. The third runs from week 28 to birth. A pregnancy is considered at term from week 37.

The scan overrides the calculation

If the first-trimester scan gives a gestational age differing from the period-based calculation by more than a few days, the estimated due date is corrected to the scan. Measuring the embryo in those weeks is highly reliable and does not depend on cycle regularity.

This does not replace medical care

An approximate calculation helps you orient yourself between appointments, not make clinical decisions.