How do mammals reproduce and develop?

Answer 1

All mammals reproduce sexually and mothers give birth to their young, except for platypuses and echidnas which lay leathery eggs but are still mammals in every other sense.

Placental mammals develop inside of their mother's womb and begin as an embryo. They gain nutrients through their umbilical cord and over time, they grow. Once born, their bodies are developed enough for the baby to live independently of the mother, although they are still dependent on the mother for milk and nurturing.

In the case of marsupials, they have a very short gestation period, and spend many more months developing within the mother's pouch. They are born extremely undeveloped, and once born, they cannot live independently of the mother. They must attach to a teat in the pouch (or the mother's underbelly in the case of the numbat, which does not have a pouch) and remain permanently attached to this teat for several weeks or months, depending on the species of marsupial. If removed prematurely, they cannot live.