Not impossibly rare, obviously. The basic general rule is if you can get through one birth alive you’re far more likely to continue to be all right. It also tends to be more dangerous for younger Clan women-- say between 20 and 50-- than for Clanswomen over that age. Beyond 1000 though, many Clan women simply have conception issues. (beyond the typical low conception rate for Clan-- women do not drop eggs every month, closer to 2-4x a year, which is closer in keeping for Tazu who only mate to conceive once a year, with half-bloods doing the 2-4x thing.)
As far as the actual children, about one in three to five will just die, either immediately after birth or within the first year. They are delicate until then, and things like even a small amount of sunlight can kill, or lack of oxygen from not Feeding enough or a variety of things. Most believe this was added into their biology by Prothidian Altar specifically to keep the population in check.
@Frogs – yes, it was impossible for me to do a deviant path for those against Semryu in WWU3. like, literally.