The short answer
Seals are pregnant for almost a year. In the harbour (common) seal the time from mating to birth is about 10.5 to 11 months; in the grey seal about 11.5 months. But the embryo only grows for part of that time: for the first two to three months it is in delayed implantation.
Delayed implantation explained
Seals mate shortly after weaning their previous pup. If the fertilised egg developed straight away, the next pup would be born at the wrong time of year. The solution is delayed implantation (embryonic diapause). The fertilised egg divides into a blastocyst of a few hundred cells and then stops growing, floating free in the uterus.
After roughly two to three months it implants in the uterine wall and true development begins, lasting around eight more months. Because mating and birth are decoupled, a female can time both to the best moment of the year. Badgers, bears and sea lions use the same trick.
The yearly rhythm in the Wadden Sea
| Stage | Harbour (common) seal | Grey seal |
|---|---|---|
| Birth | June – early July | late November – January |
| Nursing | about 3–4 weeks | about 2.5–3 weeks |
| Mating | July – August, in the water | December – January, at the end of nursing |
| Delayed implantation | about 2–3 months | about 3 months |
| Total gestation | about 10.5–11 months | about 11.5 months |
Exact timing varies by region: grey seals in Scotland or Canada, for example, pup at different times than in the Wadden Sea.
One pup a year
A seal has one pup at a time; twins are extremely rare. Females usually become sexually mature between three and five years old and can then, in principle, have a pup every year. Not every female is pregnant every year: in a poor food year or in poor condition she may skip a year.
Birth and the first weeks
Birth is quick and happens on a sandbar or beach. A newborn harbour seal has usually shed its white natal coat (lanugo) before birth and can swim within hours — necessary on Wadden sandbars that flood again at high tide. A newborn grey seal does have a white, woolly coat that is not waterproof. It stays on land for its first weeks and moults at around the end of nursing.
During the short nursing period the pup drinks extremely fat-rich milk and grows fast. Then the mother abruptly stops nursing and the pup must learn to hunt on its own. Read more on reproduction and life cycle.
Pregnant seals and disturbance
Pregnant females and mothers with pups are especially sensitive to disturbance; a mother chased off a sandbar can lose her pup. On the beach keep at least 30 metres away. Found a pup alone? Read seal pup found and, in the Netherlands, report it by calling 144.
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