Gestating

Gestation is the carrying of an embryo or fetus inside female 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 (multiple gestations).

The time interval of a gestation is called the gestation period. In human obstetrics, gestational age refers to the embryonic or fetal age plus two weeks. This is approximately the duration since the woman's last menstrual period (LMP) began.

In mammals, pregnancy begins when a zygote (fertilized ovum) implants in the female's uterus and ends once the fetus leaves the uterus.

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 to the 13th week, the second trimester is from the 14th to 27th week, and the third trimester is from the 28th week to the 42nd week.

In humans, birth normally occurs at a gestational age of about 40 weeks, though it is common for births to occur from 37 to 42 weeks. After 8 weeks, the embryo is called a fetus.

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.

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