The first trimester of pregnancy extends from conception, which occurs about 2 weeks after the last normal menstrual period, to 12 weeks after the last period: slightly less than 3 months. At the end of the first trimester, a fetus grows to about three and one-half inches long, and weighs about a half an ounce.

A fetus' heart starts beating in the middle of the first trimester. The kidneys, liver, and lungs develop during this time. The arms, hands, fingers, legs, feet, and toes also develop during this time.