ome mammals breed throughout the year, but most have distinct breeding seasons that allow birth to coincide with times when food is plentiful. In some species—for example, baboons—changes in skin color show when a female is ready to mate. However, in most mammals, males detect fertile females by sensing airborne chemicals called pheromones. The males of some mammals, including cats and horses, sense these chemicals with a receptor on the roof of the mouth called the Jacobson’s organ. As the males sniff, they curl their upper lip and seal their nostrils to draw air over the scent organ.

Young mammals are normally conceived immediately after mating. The gestation period—the time between conception and birth—varies from as little as 12 days in the case of some marsupials, to over 20 months in the African elephant. A mammal's size and its gestation period are not directly related. The tiny mouse lemur, for example, weighs about 57 g (about 2 oz), but has a gestation period of nearly 9 weeks. In a few mammal species, including rodents, bats, and kangaroos, development can be brought to a temporary halt at a very early stage. This system, called delayed implantation or embryonic diapause, allows the time of birth to be finely adjusted. In rodents, it helps to make sure that successive litters are spaced apart. In kangaroos, it allows the birth of developing young to be kept "on hold" until rain produces an abundance of food.

Mammals usually give birth on their own, but in some species—for example, dolphins and elephants—nonbreeding females sometimes gather around to help during the birthing process. Many placental mammals lick the embryonic membranes off the newborn young, and often nudge them toward the mother’s mammary glands for their first meal of milk. By contrast, female marsupials seem to pay little attention to their young, leaving them to find their own way into the pouch unaided.

Milk produced in the mother’s mammary glands provides newborn mammals with water and important nutrients. Mammals are born with a strong sucking instinct that helps them feed immediately. In monotremes, milk simply oozes out onto a patch of skin, but in marsupials and placentals, milk is released by a set of nipples or teats. Young placental mammals leave their mother's nipples between feedings, but newborn marsupials remain firmly fastened to their mother's nipples by their mouths and cannot be dislodged.

The period before weaning forms a crucial part of a young mammal's development. During this time, it plays with its siblings and learns social and survival skills from its parents. During weaning, young carnivorous mammals start to share food their parents have caught, while young plant-eating mammals begin to feed for themselves. Independence comes only when the adult teeth are fully formed, enabling the young animal to switch to an adult diet.

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