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). In the case of multiple gestations, the organisms gestated by the mother are called twins. According to the National Center for Health and Statistics there were over 136,000 multiple 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.

Humans

---

Pregnancy is divided into three, three month trimesters. The third trimester begins at about 28 weeks, 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.

In humans, parturition normally occurs at a gestational age of 37 to 42 weeks. Childbirth occurring before 37 weeks of gestation is considered preterm. Preterm and low birth weight babies make up the second leading cause of infant death at about 17%. ^*" target="_blank" > Two million babies worldwide die within 24 hours annually. [http://www.cnn.com/2006/HEALTH/parenting/05/08/mothers.index/

Dogs

---

Female dogs typically have multiple ovulations in a single estrus cycle, and, when the ova fertilize, the animal gestates multiple offspring in a single pregnancy. The average gestation period for a dog is nine weeks, regardless of breed. ^*

External links

---

• Table listing average animal gestation periods

Biological reproduction | Mammals

Gestation