The length of a pregnancy is calculated from the first day of a woman’s last menstrual period because it’s virtually impossible to know the exact moment of fertilization.
Health professionals will most likely refer to your pregnancy in terms of weeks. When they say you are “20 weeks pregnant,” for example, it means 20 full weeks have gone by since the first day of your last menstrual period. The reason is simple: it is more accurate to talk about weeks than calendar months.
Your baby will be considered at term as of 37 weeks and could be born anytime between 37 and 42 weeks after your last menstrual period. Most babies are born between 39 and 41 weeks.
The 42 weeks of pregnancy (maximum length) are further divided into three trimesters of 14 weeks each. Each trimester corresponds to specific stages in the development of the fetus (see Development of the fetus).
Fertilization: Fusion of a sperm and egg.
Fetus: Developmental stage of a human being in its mother’s womb, from 10 weeks of pregnancy until birth.