Gestation 12312 225875825 2008-07-15T20:32:48Z 69.14.192.205 /* Humans */ '''Gestation''' is the carrying of an [[embryo]] or [[fetus]] inside a [[female]] [[viviparous]] [[animal]]. Mammals during [[mammalian pregnancy|pregnancy]] can have one or more gestations at the same time ([[multiple birth|multiple gestations]]). The time interval of a gestation plus 2 weeks is called ''gestation period'', and the length of time plus 2 weeks that the offspring have spent developing in the uterus is called ''[[gestational age]]''. (The extra 2 weeks is because gestational age is counted starting from the last menstrual period (LMP), rather than actual conception. However this extra 2 weeks is not always added when talking about animals.) ==Humans== {{main|Pregnancy}} Human pregnancy can be divided into three trimesters, each three months long. The First Trimester is from 0 to 12 weeks, The Second Trimester is from 12 to 28 weeks & The Third Trimester begins at about 28 weeks. 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". <ref> [http://www.marchofdimes.com/pnhec/188_1080.asp March of Dimes - Preterm labor and birth]</ref> Preterm and low birth weight babies make up the second leading cause of infant death at about 17%. {{Fact|date=February 2007}} Preterm births solely consist of 12% of infant deaths with an 84% majority within the 32-36 week period. <ref> [http://www.marchofdimes.com/professionals/681_1157.asp March of Dimes - Preterm Birth]</ref> It is estimated that two million babies worldwide die annually within 24 hours of birth. ==Mammals== {{main|Pregnancy (mammals)}} In mammals, pregnancy begins when a [[fertilisation|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 (note for humans gestational age is counted from the LMP, for other animals the counting method varies, so these figures could be 14 days off): {| class="wikitable" |- ! Animal !! Average gestation period (days) |- | [[Rabbit]]s | 33 |- | [[Cat]]s | 62 |- | [[Dog]]s | 65 |- | [[Lion]]s | 108 |- | [[Pig]]s | 115 |- | [[Sheep]] | 150 |- | [[Human]]s | 259-294 |- | [[Cattle]] | 283 |- | [[Horse]]s | 336 |- | [[Elephant]]s | 600-660 |} ==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 (biology)|egg]] ([[ovipary]]). The mother then gives [[live birth]]. The less developed form of vivipary is called [[ovoviviparity]], which, for instance, occurs in most [[Viperidae|vipers]]. The more developed form of vivipary is called [[placenta|placental viviparity]]; mammals are the best example, but it has also evolved independently in other animals, such as in [[scorpion]]s, some [[shark]]s, and in [[velvet worm]]s. 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 [[animal]]s develop within [[Egg (biology)|egg]]s that remain within the mother's body up until they hatch or are about to hatch. This strategy of [[childbirth|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 [[embryo]]s 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 [[shark]]s, [[reptile]]s, and [[invertebrate]]s. The young of ovoviviparous [[amphibians]] are sometimes born as [[larva]]e, and undergo [[Metamorphosis (biology)|metamorphosis]] outside the body of the mother. The [[Syngnathidae]] family of fish has the unique characteristic where females lay their [[egg (biology)|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 [[seahorse]]s, the [[pipefish]], and the [[weedy sea dragon|weedy]] and [[leafy sea dragon]]s. Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy" has been applied.<ref name="biology>{{cite journal | last = Jones | first = Adam G. | coauthors = Avise, John C. | title = Male Pregnancy | journal = Current Biology | volume = 13 | issue = 20 | pages = R791 | date = [[2003-10-14]] | url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VRT-49SN5B1-5&_user=10&_handle=V-WA-A-W-WD-MsSAYWW-UUA-U-AAZAEZDBBD-AACECVYABD-ACYAVUUBU-WD-U&_fmt=full&_coverDate=10%2F14%2F2003&_rdoc=18&_orig=browse&_srch=%23toc%236243%232003%23999869979%23464936!&_cdi=6243&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=08311121da42f6825bc092a851bda224 | format = [[HTML]] | doi = 10.1016/j.cub.2003.09.045 }}</ref> ==References== {{Reflist}} ==See also== *[[Pregnancy]] (humans) *[[Pregnancy (mammals)]] *[[Nesting instinct]] *[[Prenatal development]] ==Further reading== *[http://www.babycenter.com/comments/baby/postpartumbeauty/2432 Support Forum for Labour] *[http://www.infoplease.com/ipa/A0004723.html Table listing average animal gestation periods] *[http://pregnancy-articles.com/pregnancy-due-date.html Pregnancy Due Date] [[Category:Reproduction]] [[Category:Mammals]] [[bg:Бременност]] [[es:Gestación]] [[fr:Gestation]] [[hr:Gestacija]] [[simple:Gestation]] [[vi:Thai]]