They give birth to as many as 18 young. The little ones, no larger than a honeybee, are born before the development stage of eutherians. Like all marsupials, immediately following birth newborn opossums continue to form inside the mother's pouch, where each receives nourishment from a nipple. They all remain inside this pouch for over a month. Not all mammals are born alive.
Monotremes are a subclass of mammals.
The mothers lay one or two small eggs and incubate them against their body for around two weeks. Once the eggs hatch, the young are nourished with milk, but not from a nipple or teat like other mammals. The milk is secreted from the glands onto the mother's belly where the newborn slurps instead of sucks. There are no monotremes living in Pennsylvania. Of all the mammals found in the wild throughout Pennsylvania, the largest is the elk.