Embryonic diapause

Embryonic diapause is a reproductive strategy used by close to 100 different mammals in seven different orders. In embryonic diapause, the embryo (blastocyst) does not immediately implant in the uterus, but is maintained in a state of dormancy. No development takes place as long as the embryo remains unattached to the uterine lining. As a result, the normal gestation period is extended, sometimes up to a year. 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. 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.

Purpose
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 to a female's benefit to have ideal conditions (e.g. available food, mild weather, previous offspring weaned) to ensure the survival of her offspring before giving birth. 

Types
Two types of embryonic diapause have been identified.

Facultative diapause
Facultative diapause is a mechanism that is associated with metabolic stress, normally lactation. If a female copulates while still lactating for her original offspring, the sucking stimulus will cause the embryos to enter into diapause. This is known to occur in some rodents, insectivores and marsupials. Facultative diapause can also be referred to as facultative arrest. Another example of diapause associated with metabolic stress associated is that exhibited by the brine shrimp, Artemia franciscana. The adult female which usually gives birth to live offspring (nauplii) desists and instead produces encysted embryos in the fall season which is characterized by low temperature and high salinity. These embryos are known to have very low metabolic activities and can survive up to 17 months of anoxia (complete lack of oxygen). In favorable conditions, the 'cysts' will hatch and release nauplii to continue their life cycle.

Artemia franciscana cyst: measuring roughly 200 - 250 micrometers

Obligate diapause
Obligate diapause 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 many of the pinnipeds, mustelids, ursids, armadillos, one species of fruit bat, and the roe deer. Obligate diapause can also be referred to as obligate arrest.

For example, the brown bear will mate in early summer and the embryo will float freely in the uterine horns until the female hibernates. At this time the embryo will attach to the wall of the uterus. Eight weeks later the cub is born. It is born early, presumably to save energy (if it remained internally it would require the mother to convert her fat reserves to glucose, but if it is born it can get the fat as glycogen directly from the mothers milk).

Remarks
Embryonic diapause can also be referred to as delayed implantation.

Diapause is a term used to refer to the temporary partial or complete metabolic suspension in various life stages of insects and plants.