Gestation is the period of development during the carrying of an embryo or fetus inside viviparous animals. It is typical for mammals, but also occurs for some non-mammals. Mammals during pregnancy can have one or more gestations at the same time, for example in a multiple birth.
The time interval of a gestation is called the gestation period. In human obstetrics, gestational age refers to the fertilization age plus two weeks. This is approximately the duration since the woman's last menstrual period (LMP) began.
On the main article link above, are average and approximate gestation values ordered by number of days (note: human gestational age is counted from the last menstrual period; for other animals the counting method varies, so these figures could be 14 days off)
Human pregnancy can be divided roughly into three trimesters, each approximately three months long. The first trimester is from the last period through the 13th week, the second trimester is 14th-27th week, and the third trimester is 28th-42nd week. Birth normally occurs at a gestational age of about 40 weeks, though it is common for births to occur from 37 to 42 weeks. From the 9th week of pregnancy (11th week of gestational age), the embryo is called a fetus.
Various factors can come into play in determining the duration of gestation. For humans, male fetuses normally gestate several days longer than females and multiple pregnancies gestate for a shorter period.
A viviparous animal is an animal employing viviparity: the embryo develops inside the body of the mother, as opposed to outside in an egg (oviparity). The mother then gives live birth. The less developed form of viviparity is called ovoviviparity, which, for instance, occurs in most vipers. The more developed form of viviparity 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. The young of ovoviviparous amphibians are sometimes born as larvae, and undergo metamorphosis outside the body of the mother.
The fish family Syngnathidae has the unique characteristic whereby 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.