Females and unweaned offspring stay together, and the sexes come in contact during breeding season, late February and March in Pennsylvania. After mating, the female drives off the male. The male plays no part in raising young. Newborn young are hairless, pink-skinned, blind and scarcely past the embryonic stage. They are about one-half inch in length and weigh 0.005 ounces.
Hind limbs are rudimentary, but the front limbs and feet are well-developed and equipped with claws.
The young crawl upward, with overhand strokes as if swimming, through the mother's fur to a pouch in the skin of her belly. The pouch is lined with fur and contains mammary glands. When a young opossum attaches and begins to nurse, the nipple enlarges, forming a bulb on the end which swells in the baby's mouth and helps it stay attached. The female usually has 13 mammary glands, so offspring in excess of this number die. The mother can close her pouch to keep the young from falling out.