Mammals share some distinct characteristics. All mammals are vertebrates, warm blooded, they have body hair or fur, breathe using lungs, and nourish their young with milk. Mammals give birth in three ways: placental, marsupial, and monotreme.
The placenta is an organ that provides oxygen and nutrients from the mother, and removes excrement. The placenta enables placental mammals to survive far longer inside the uterus than other mammals. Blue whales can spend an entire year inside the uterus. When giving birth the umbilical cord tears and the newborn's own respiratory, circulatory, and waste disposal systems activates. Once outside the womb, the infant drinks milk from the mother's nipples. Placental mammals include humans, dogs, giraffes, and many more.
Marsupials are born very small. Kangaroo babies are only the size of a jellybean. The quoll are a marsupial that is born only weighing 18mg. They are born not fully developed, so they continue growing inside a pouch on the mother where they can drink milk from nipples. Some marsupials can 'pause' ther pregnancies in unfavorable conditions. Kangaroos can ever produce different types of milk at the same time for infants of different ages.
There are only five species of monotremes left on Earth. This includes four species of echidnas and the platypus. Monotremes use one orifice to reproduce, lay eggs, and dispose of waste. The eggs are soft shelled, and when the baby is born it suckles from milk secreted from pores on the mothers body.