Pregnancy Week Calculator

How Pregnancy Weeks Are Counted

Gestational age is counted from the first day of your last menstrual period, not from conception. This convention exists because the date of the last period is usually known while the exact date of conception often is not. It means that during the first two weeks of "pregnancy" you are not actually pregnant yet; ovulation typically occurs around week two of the count.

Because of this convention, gestational age runs about two weeks ahead of fetal age. When a provider says you are 12 weeks pregnant, the embryo has been developing for roughly 10 weeks.

Weeks and Days

Pregnancy is usually expressed as weeks plus days, for example 24 weeks and 3 days, often written as 24+3. This is more precise than rounding to whole weeks and matters for scheduling tests and appointments. The calculator shows your result in this format.

The Three Trimesters

• First trimester (weeks 1–13): the period of fastest development, when major organ systems form.
• Second trimester (weeks 14–27): often described as the most comfortable stage, with growth and movement becoming noticeable.
• Third trimester (weeks 28–40): continued growth and maturation in preparation for birth.

Full term is generally defined as 39 to 40 weeks, with births from 37 weeks considered early term. These categories inform clinical care rather than predicting any individual outcome.

Why the Estimate Can Shift

A calculation from your last period assumes you ovulated on day 14 of a 28-day cycle, which is not true for everyone. An early ultrasound measurement is generally more accurate and may revise the gestational age. If your provider records a different date, that date takes precedence over this estimate.