Mammals that give birth to fully developed live young are termed

Viviparous

Explanation:

I think that's the word you are looking for

Viviparous animals give birth to live offspring after they have developed in the parent's body

The opposite of this would be Oviparous, meaning an animal that lays eggs. (birds, etc)

Fun fact:

the echidna (also known as a spiny anteater) and the duck-billed platypus are the only mammals known to lay eggs!