What is embryonic diapause?

It's a delay in embryonic development between the initial fertilization and the time the developing early embryo actually implants itself into the uterus or, in non-mammalian species, egg development.

Some species use this mechanism to ensure a favorable birth time for their young when the normal pattern of the mating season plus gestation period would not guarantee this timing. As an example, the maturation of an embryo or egg could be delayed until the outside temperature reaches a certain level, ensuring the young don't get born / hatched into wintery conditions.

https://en.wikipedia.org/wiki/Embryonic_diapause