Delayed implantation or embryonic diapause is a reproductive strategy used by approximately 100 different mammals in seven or eight different orders. In embryonic diapause, the embryo (blastocyst) does not immediately implant in the uterus after sexual reproduction has created the zygote, but is maintained in a state of dormancy. Little to no development takes place while the embryo remains unattached to the uterine wall. As a result, the normal gestation period is extended for a species-specific time. While much of the molecular regulation involved in activating dormant blastocysts has been characterized, little is still known about entry into diapause, and the conditions which enable a blastocyst to remain dormant.
Some mammals that undergo embryonic diapause include rodents, bears, mustelids (e.g. weasels and badgers), and marsupials, (e.g. kangaroos). Some groups only have one species that undergoes embryonic diapause, such as the roe deer in the order Artiodactyla.
Mammals may undergo diapause to avoid the risk to their own lives during unfavourable and/or environmental conditions. Mammals use embryonic diapause to time the birth of their offspring for favorable metabolic and/or environmental conditions. Reproduction has a large energy cost and it is beneficial to have ideal conditions (e.g. available food, mild weather, previous offspring weaned) to ensure the offspring survives before giving birth.
Two types of mammalian embryonic diapause have been identified.
Facultative diapause is also known as lactational delayed implantation due to its regulation via lactation. If a female copulates while still lactating for a previous litter, the suckling stimulus will cause the embryos to enter into diapause. This is known to occur in some rodents, insectivores and marsupials.
Obligate diapause is also known as seasonal delayed implantation and is a mechanism that allows mammals to time the birth of their offspring for favorable environmental conditions. This mechanism occurs as a regular part of the reproductive cycle in mammals such as armadillos, all species of pinniped, many mustelids, all ursids, one species of fruit bat, and the roe deer.