During a full‐term human pregnancy,
a fetus is carried in the uterus for a gestation
period averaging 280 days from the last menstrual period until birth (266 days from conception to birth). The 280 days are frequently divided into 10 four‐week lunar months
of 28 days each, which equal slightly more than nine calendar months,
as shown in Figure . Pregnancies are also often divided into trimesters, periods of three calendar months (13 weeks).