There's been a show running on SBS on a Sunday night called Inside Nature's Giants, which involves dead animal dissection to learn about their odd biological features.
Last night, it was the kangaroo's turn (even though I would hardly think they count as "giants"), but in any event I was reminded about the odd feature of how female kangaroos can keep an embryo in stasis in their uterus (of which they have two, as well as three vaginas) while they have a joey in the pouch.
I was wondering how much is known about how the biology of that works, but Googling is not showing up all that much information on the topic. Embryonic diapause has its own Wikipedia entry, but it's pretty brief. It does show, though, that quite a lot of mammals can do this trick.
The whole topic reminded me of a later Heinlein novel, in which the heroine turns out to have been secretly carrying an embryo, at body temperature of course, in a small genetically engineered "pouch" in her navel. I think it must have been Friday, but even that has little information on the Web. Anyhow, I remember thinking at the time that body temperature stasis of a human embryo seemed a bit unlikely, but I don't recall if at the time I realised that there were local mammals doing this trick.
I wonder how much biological study this has ever received. It would be a good trick if it could be applied to human embryos, in lieu of freezing them.