Answer: Kangaroos

It’s such a curious concept that you may have never even considered it could actually be done in the first place. In humans, there is no pausing a pregnancy: thrilled with the news or devastated, the development of the pregnancy chugs along for approximately nine months and along comes a baby. If you have a particularly stressful stretch of projects at work, you don’t get the option to pause the development and resume it when the stress passes and life resumes a more balanced state.

Yet that’s the exact reproductive strategy employed by kangaroos. Kangaroos are prolific breeders and female kangaroos are, more or less, permanently pregnant for the duration of their fertile years. Given the limited amount of energy and resources to care for their offspring, however, constant pregnancy could prove problematic. As such, they use a clever adaptation known as “embryonic diapause” to ensure maximum success of their offspring.

The female kangaroo can, after becoming pregnant, effectively “pause” the process and put the embryo into a state of dormancy where it remains viable, but does not implant and continue developing. Once environmental stressors like drought and famine have passed or the previous joey leaves the pouch, the embryo ceases being dormant and completes the implantation process to resume the pregnancy.

Although kangaroos might be the most notable example of this phenomenon because of their other motherhood-adaptations (such as short gestational periods where the joey lives in the mother’s pouch to finish growing and the ability to produce different kinds of milk for joeys of different ages still in her pouch), there are roughly 100 known examples of diapause in mammals.