The first trimester of pregnancy typically lasts from the start of pregnancy (counted from the first day of the last menstrual period) up to the end of the 12th or 13th week. Most sources agree that it covers roughly the first 12 to 13 weeks of pregnancy, or about three months

. Specifically:

• The Health Service Executive (HSE) states the first trimester is from 0 to 12 weeks

• Cleveland Clinic and Geisinger both define it as lasting through the end of week 13, or about three months

• The Better Health Channel and NHS also mark the first trimester as conception to 12 weeks

• Jackson Health notes a trimester lasts between 12 and 14 weeks, with the first trimester going up to 13 weeks

• The Bump cites the American College of Obstetricians and Gynecologists (ACOG) definition as ending at 13 weeks and 6 days

In summary, the first trimester is generally considered the first 12 to 13 full weeks of pregnancy.