Pregnancy is the period of time between fertilization of the ovum (conception) and birth, during which mammals carry their developing young in the uterus (see embryo). The duration of pregnancy in humans is about 280 days, equal to 9 calendar months. After the fertilized ovum is implanted in the uterus, rapid changes occur in the reproductive organs of the mother. The uterus becomes larger and more flexible, enlargement of the breasts begins, and alteration of renal function, blood volume, and blood cell count occur. Movement of the fetus and fetal heartbeat can be detected early in pregnancy.
