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 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 the last period to the 13th week, the second trimester is from the 14th to 27th week, and the third trimester is from the 28th week through the 40th week.American College of Obstetricians and Gynecologists - How Your Baby Grows During Pregnancy

In humans, birth normally occurs at a gestational age of about 40 weeks, though a normal range is from 37 to 42 weeks. Childbirth occurring before 37 weeks of gestation is considered preterm, whereas childbirth after 42 weeks is considered postterm. 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. 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 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):

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.