A normal pregnancy lasts about 40 weeks from the first day of your last menstrual period to the birth of a baby. This time is roughly divided into 3 periods: the first trimester, from the first day of your last menstrual period to about the 12th week of pregnancy; the second trimester, from about 13 to 27 weeks of pregnancy; and the third trimester, from about 28 weeks of pregnancy until birth.

Each trimester of pregnancy is marked by developmental changes in the fetus.

• By the end of the first trimester (about week 12 of pregnancy), the fetus has a recognizable human form.
• By the end of the second trimester (about week 27 of pregnancy), all the vital organs of the fetus have developed. Also, the mother begins to feel the fetus begin to move (quickening), usually starting between weeks 16 and 20.
• During the third trimester (about week 28 until birth), the fetus's size increases and organs mature.