Difference between revisions of "Gestation" - New World Encyclopedia
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==Overview== | ==Overview== | ||
| − | Viviparous animal are those where the embryo develops inside the body of the mother, as opposed to outside in an [[Egg (biology)|egg]] ([[ovipary]]) | + | Viviparous animal are those where 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]], where the animals develop within eggs that remain within the mother's body up until they hatch or are about to hatch. The embryos are nourished by the egg yolk, rather than the mother's body. Ovoviviparity is employed by many aquatic life forms such as [[fish]] and some [[shark]]s, [[reptile]]s, and [[invertebrate]]s, such as [[scorpion]]s. Ovoviviparity occurs for example in most [[Viperidae|vipers]]. In other forms of vivipary, such as placental vivipary, the embryo is nourished by the mother's body and is not encased in a egg. In placental viviparity, as with humans, there is a placenta connecting the mother with the developing fetus. | The less developed form of vivipary is called [[ovoviviparity]], where the animals develop within eggs that remain within the mother's body up until they hatch or are about to hatch. The embryos are nourished by the egg yolk, rather than the mother's body. Ovoviviparity is employed by many aquatic life forms such as [[fish]] and some [[shark]]s, [[reptile]]s, and [[invertebrate]]s, such as [[scorpion]]s. Ovoviviparity occurs for example in most [[Viperidae|vipers]]. In other forms of vivipary, such as placental vivipary, the embryo is nourished by the mother's body and is not encased in a egg. In placental viviparity, as with humans, there is a placenta connecting the mother with the developing fetus. | ||
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However, in [[human]]s, the term embryo is precisely defined. It is reserved for the developing organism from conception and implantation until the first eight weeks (sometimes seven weeks), after which it is called a fetus. At eight weeks, the human being is not fully developed nor able to survive as a separate organism. The fetus then is defined as from eight weeks (or seven weeks) after fertilization until birth. | However, in [[human]]s, the term embryo is precisely defined. It is reserved for the developing organism from conception and implantation until the first eight weeks (sometimes seven weeks), after which it is called a fetus. At eight weeks, the human being is not fully developed nor able to survive as a separate organism. The fetus then is defined as from eight weeks (or seven weeks) after fertilization until birth. | ||
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==Humans== | ==Humans== | ||
Revision as of 00:16, 19 September 2008
Gestation is the period of development inside a female viviparous species of one or more offspring; that is, a time period involving the carrying of an embryo or fetus inside the uterus or female reproductive organ of a viviparous animal. The time of gestation varies greatly among animals. For example, the European rabbit has a gestation length of about 30 days, and they produce about five to seven liters per year (Angerbjörn 2004). The gestation period for an elephant is about 22 months, the longest for any land animal. Gestation in marsupials tends to be shorter than that of placentals (the marsupial cat, Dasyurus viverrinus has a gestation period of only eight days) as the immature young are raised in a pouch after birth until early infancy.
- All living beings go through stages of development, requiring the passage of time. The embryo is one part of the formation stage for the development of an organism. It comes on the foundation of the fertilization of gametes and prepares the ground work for the next stage, the fetus of a human or, in the case of a plant, seedling.
The human gestation period is about 38 weeks from fertilization (conception), or approximately 40 weeks from the the last normal menstrual period (LNMP) in humans. Thus, it takes about nine months.
Overview
Viviparous animal are those where 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, where the animals develop within eggs that remain within the mother's body up until they hatch or are about to hatch. The embryos are nourished by the egg yolk, rather than the mother's body. Ovoviviparity is employed by many aquatic life forms such as fish and some sharks, reptiles, and invertebrates, such as scorpions. Ovoviviparity occurs for example in most vipers. In other forms of vivipary, such as placental vivipary, the embryo is nourished by the mother's body and is not encased in a egg. In placental viviparity, as with humans, there is a placenta connecting the mother with the developing fetus.
Gestation is the time period, between fertilization and the birth of the offspring, when the mother is carrying the offspring in her body. This time period can be further divided into embryo and fetus. The term embryo sometimes is used for the time period of an organism at any time after fertilization and before birth or hatching. Others limit the term to the time between fertilization and when it is anatomically and physiologically complete (fully developed) and able to survive as a separate organism. After this comes the fetus, which is the time period immediately after the embryo stage up until birth. The starting point at which a fetus begins may be imprecise because the exact time period corresponding to the embryonic stage generally is imprecisely defined.
However, in humans, the term embryo is precisely defined. It is reserved for the developing organism from conception and implantation until the first eight weeks (sometimes seven weeks), after which it is called a fetus. At eight weeks, the human being is not fully developed nor able to survive as a separate organism. The fetus then is defined as from eight weeks (or seven weeks) after fertilization until birth.
Humans
Human pregnancy can be divided into three trimesters, each three months long. The First Trimester is from conception (the 0th week) to the 12th week, The Second Trimester is from the 13th to 28th week & The Third Trimester is from the 29th week through the 36th week.
In human beings, 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.)
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 24 weeks is usually considered "viable". [1] 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. [2] It is estimated that two million babies worldwide die annually within 24 hours of birth.
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 (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):
| Animal | Average gestation period (days) |
|---|---|
| Rabbits | 33 |
| Cats | 62 |
| Dogs | 65 |
| Lions | 108 |
| Pigs | 115 |
| Sheep | 150 |
| Humans | 259-294 |
| Cattle | 283 |
| Horses | 336 |
| Elephants | 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 (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.[3]
ReferencesISBN links support NWE through referral fees
- ↑ [1]
- ↑ March of Dimes - Preterm Birth
- ↑ Jones, Adam G. and Avise, John C. (2003-10-14). Male Pregnancy. Current Biology 13 (20): R791.
- Angerbjörn, A. 2004. Hares and rabbits (Leporidae). Pages 505 to 516, in B. Grzimek et al. Grzimek's Animal Life Encyclopedia. Detroit, MI: Thomson/Gale. ISBN 0787657921
See also
- Pregnancy (humans)
- Pregnancy (mammals)
- Nesting instinct
- Prenatal development
Further reading
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