I'm still most fascinated by Zuul biology, since after the comprehensive writeups on the other races I know
that it's all been worked out. My thinking on their birthing process (and this seems to dovetail with what Mecron's been saying) is that they can probably give birth as mammals do; they do
look like mammals (which obviously doesn't necessarily
mean anything), they appear (from what little we've seen) to have small litter sizes, and their females appear to be large, fully-developed members of the species, all of which sort of argue against "lethal auto-caesarean" as the standard delivery method. I suspect that they would normally give birth with less deleterious effects on the mother, but that under certain circumstances (apparently, at least when the mother dies) viable young can free themselves in a fairly gory fashion.
Edit: I kind of wonder if they have some similarities to terrestrial marsupials, for which the young move to an external pouch when they reach a certain level of development, where they can continue to develop to a significant extent. Obviously in the case of the Zuul, such a secondary stage of gestation for viable young wouldn't be in an external
pouch, since the newborns in the story had to burrow their way out. But the fact that Zuul females (or just Zuul if they're hermaphrodites/asexual and the marine's assessment was wrong) can carry within their bodies young that are sufficiently developed to claw/bite their way into the world where they can then survive on their own makes me wonder if they have some sort of roughly analogous specialized gestation.
I was also wondering how the young would sense that it was necessary to "deliver" themselves like those in the story, but then I remembered that Zuul are telepathic.
They could have all sorts of biological triggers, delivered consciously or unconsciously, that are just not available to Earth species.