Delayed implantation is a phenomenon that lengthens the gestation period of many mammals. The blastocyst (a stage in embryo development a little beyond fertilization) remains dormant, and its implantation in the uterine wall is postponed for a period of time lasting from a few weeks to several months. Many mammals (including bears, seals, weasels, badgers, bats, and some deer) use this to extend their gestation period through delayed implantation so they give birth at the time of year that offers the best chance of survival for their young.