Seal pups are born on land in the spring and summer. To take advantage of warmer seasonal environments and plentiful food, some seal species are migratory, feeding in one spot in the summer and early autumn, and then traveling to a warmer spot in the autumn and winter to give birth and mate shortly afterwards. Seals can give birth in large groups, in which a crowd of seals have returned to a particular spot to breed, or they can give birth alone. Migratory seals usually give birth in groups, after which they mate with males and conceive another pup.
Another way to ensure that a pup is born at an optimal time is to delay implantation of the embryo inside the uterus. In seals, fertilization (the meeting of egg and sperm) may take place in April, but the embryo might not implant in the mother's uterus until October. This phenomenon of delayed implantation also occurs in roe deer, armadillos, and badgers. The total gestation period (the time it takes for the pup to develop inside its mother) is 9-15 months, depending on the species. The average active gestation period (the time from implantation to birth) is probably about 3-5 months.
Science EncyclopediaScience & Philosophy: Jean-Paul Sartre Biography to Seminiferous tubulesSeals - The Body, Reproduction, Diversity - Seals are mammals, General characteristics of seals, Diving and reproduction