Gestation

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). 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

Main article: Pregnancy

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". March of Dimes - Preterm labor and birth Preterm and low birth weight babies make up the second leading cause of infant death at about 17%.^{[citation needed]} Preterm births solely consist of 12% of infant deaths with an 84% majority within the 32-36 week period. March of Dimes - Preterm Birth It is estimated that two million babies worldwide die annually within 24 hours of birth.

Mammals

Main article: Pregnancy (mammals)

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:

Non-mammals

A viviparous animal is an animal employing vivipary: the embryo develops inside the body of the mother, as opposed to outside in an egg (ovipary). The mother then gives live birth. The less developed form of vivipary is called ovoviviparity, which, for instance, occurs in most vipers. The more developed form of vivipary is called placental viviparity; mammals are the best example, but it has also evolved independently in other animals, such as in scorpions, some sharks, and in velvet worms. Viviparous offspring live independently and require an external food supply from birth. Certain lizards also employ this method such as the genera Tiliqua and Corucia. The placenta is attached directly to the mother in these lizards which is called viviparous matrotrophy.

Ovoviviparous animals develop within eggs that remain within the mother\'s body up until they hatch or are about to hatch. This strategy of birth is known as ovoviviparity. It is similar to vivipary in that the embryo develops within the mother\'s body. Unlike the embryos of viviparous species, ovoviviparous embryos are nourished by the egg yolk rather than by the mother\'s body. However, the mother\'s body does provide gas exchange. Ovoviviparity is employed by many aquatic life forms such as fish and some sharks, reptiles, and invertebrates. The young of ovoviviparous amphibians are sometimes born as larvae, and undergo metamorphosis outside the body of the mother.

The Syngnathidae family of fish has the unique characteristic where females lay their eggs in a brood pouch on the male\'s chest, and the male incubates the eggs. Fertilization may take place in the pouch or before implantation in the water. Included in Syngnathidae are seahorses, the pipefish, and the weedy and leafy sea dragons. Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy" has been applied.Jones, Adam G.; Avise, John C. (2003-10-14). "Male Pregnancy" (HTML). Current Biology 13 (20): R791. 

References

See also

• Pregnancy (humans)
• Pregnancy (mammals)
• Nesting instinct
• Prenatal development

Further reading

• Support Forum for Labour
• Table listing average animal gestation periods
• Pregnancy Due Date

This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia