Sea otter

They are polygynous, males have several female partners.

An excerpt from the article 19 facts about Sea otter

Their gestation period can vary greatly. It can last from four months to a year after copulation. This is due to delayed implantation, a strategy that allows the fertilized egg not to implant in the uterus immediately after fertilization, but to enter a pause (diapause) for a while before it starts to develop further.

This strategy allows the mother to time the birth of her offspring with the most advantageous conditions for survival.