Female harp seals are ready to mate as soon as they have finished nursing their pups, about 12 days after giving birth. Toward the end of lactation, males begin to circulate through the whelping patch searching for a mate. Harp seals mate promiscuously, without any prolonged pair-bonding. Although courtship behaviour and mating attempts occur on the ice, mating usually takes place in the water. The fertilized egg divides to form an embryo, which floats freely in the womb for about three and a half months before it implants in the wall of the uterus. This process is known as delayed implantation, and assures that all females will give birth to their pups at about the same time each year. This timing is important for a species such as the harp seal, which depends on short-lived pack ice as a whelping substrate.

The entire process of birth, nursing, and mating for the following year is accomplished in about two weeks, which is very fast for such a large mammal. The rapid development of pups allows for a short period of maternal care, which minimizes the time female seals are tied to the ice, where they are more vulnerable to predators.