Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time (multiple gestations). According to the National Center for Health and Statistics there were over 136,000 multiple human births documented in the United States in 2003(NCHS).  The time interval of a gestation is called gestation period, and the length of time the offspring have spent developing in the uterus is called gestational age.
Human pregnancy can be divided into three trimesters, each three months long. The third trimester begins at about 28 weeks. The 23rd week is the first week when a preterm fetus is considered viable. Before this age major developmental events that would allow the fetus to survive outside the womb have not yet occurred. This division is somewhat arbitrary as children born before this point have survived, but only with significant medical assistance.
In humans, birth normally occurs at a gestational age of 37 to 42 weeks. Childbirth occurring before 37 weeks of gestation is considered preterm, childbirth after 25 weeks is usually considered "viable".  Preterm and low birth weight babies make up the second leading cause of infant death at about 17%. Preterm births solely consist of 12% of infant deaths with an 84% majority within the 32-36 week period.  It is estimated that two million babies worldwide die annually within 24 hours of birth.
In mammals, pregnancy begins when a fertilized zygote implants in the female's uterus; and ends once it leaves the uterus.
Below are average and approximate values ordered by gestation period:
|Animal||Average gestation period (days)|