Mammals are usually viviparous.

The process in which an animal's fertilized egg matures and produces in the uterus of a female animal is called viviparous. Nutrients needed for embryonic development can be obtained from the mother until birth.

Embryos absorb nutrients and oxygen in maternal blood through placenta and umbilical cord during development, and at the same time transport metabolic waste to the mother. After the fetus develops in the mother's womb, it is directly delivered by the birth canal.

The process of viviparity

Generally, the fertilized eggs of viviparous animals are small. They are fertilized at the upper end of the mother's fallopian tube, and then develop into early embryos and implant in the uterus. After that, they grow on the inner wall of the mother's uterus, connect with the mother through the placenta, absorb nutrients and oxygen in the mother's blood, transport carbon dioxide and waste into the mother's blood, and discharge through the mother.

When the fetus matures, the uterus contracts to expel the larvae from the body, forming an independent new life. All mammals are viviparous, only platypus and echidna are oviparous.

The above contents refer to Baidu Encyclopedia-viviparous.