Delayed implantation

Delayed implantation is a physiological adaptation to pregnancy in some mammals. Without it, a fertilized egg in the female body almost immediately attaches to her uterus and starts developing into a fetus. With delayed implantation, the fertilized egg stays inactive for several weeks or even months before it attaches and starts to develop. The female puts her pregnancy “on hold” for a while.

With delayed implantation, the female gains some flexibility in how long she stays pregnant (as opposed to for example humans, who by nature do not have delayed implantation). It is generally important for wild animals to give birth at a specific time of the year, typically when spring is warm enough. With delayed implantation, the time of birth does not so much depend on the time of mating. This allows females to spend longer times on searching for the perfect mate.

Brown bears may put their pregnancy “on hold” right after fertilization to make sure their offspring is born at a time of suitable conditions. Photo: Paul E. Aspholm, NIBIO