... of a
blastula into the
uterus can mean the difference between life and death to a would-be mother.
For female mammals, pregnancy carries a huge energy cost. If one doesn't have the necessary resources to carry her pregnancy to term, there's a risk of losing both the unborn offspring and a formerly healthy mother. This is extremely bad in evolutionary terms because it may take many successful pregnancies to produce even one healthy adult!
Certain public figures would suggest abstinence as an alternative to this danger, but many species have such brief breeding seasons that abstaning has its own reproductive cost: How many of us would give that pragmatic "no" if we knew we wouldn't so much as see a member of the opposite sex for 18 months? /end digression.
Enter delayed implantation. After fertilization, many mammal females are able to carry an unimplanted blastula in their uterus without entering a pregnant state. During this time, they can collect resources and pick up on environmental clues to better assess the chances of a successful childbirth. If conditions are still unsuitable for too long, her body will biochemically abort the blastula, preventing the risk. If she makes it to the right season, the right weight, or the right company, the blastula is allowed to implant and gestation begins.
In Roe deer, this is used to artificially delay a short gestation period so that summer matings give spring births. Elephant Seals also use it, for both birth timing and adaptation to uncertain food supplies.