The common seal reproduces through live birth, with the female giving birth to one pup on land or ice. The gestation period is 8 months and 2 weeks, and the female has one pup per litter. Young harbor seals are fed milk for approximately four weeks, and reproduction occurs once a year. The interval between reproductions ranges from 11 months to 1 year. The species inhabits and develops on land, outside of bodies of water, with eggs, larvae, or young developing in nests built on the ground or close to it. These nests are located directly on the surface of the ground. Individuals reach sexual maturity from 3 to 6 years of age, and reproduction always involves fertilization without generational change.