Birth is the process by which a fully developed fetus is expelled from the mother's uterus by the force of strong, rhythmic muscle contractions.

Usually lasting about 16 hours—but in some cases ranging from less than one hour to 48 hours—birth is the culmination of a series of complex physical processes. The length of time between fertilization and birth is called the gestation period. In humans, the gestation period is approximately nine months or 38 weeks.

Birth typically begins at the end of the gestation period when the mother's uterus begins to contract rhythmically, a process called labor. The initiation of labor leading up to birth is the result of a number of hormones, notably oxytocin. Oxytocin is a hormone released from the pituitary gland in the brain, which stimulates uterine contractions and also controls the production of milk in the mammary glands of the breast (a process called...