As far as we know only one Vampire Slayer has ever been a mum, and that was Nicky, the Seventies afro-ed black Slayer whose son Robin turns up in Season 7, as the headmaster of the new Sunnydale High.
What I want to know is, how on earth did Nicky manage?
Firstly, there are all sorts of physical limitations on any mother-to-be during pregnancy. Did Nicky just take it easy on the patrolling side of things for nine months? It wouldn't have been easy training while suffering from morning sickness, for instance, and her doctor's instructions to avoid heavy lifting would surely preclude her from most slaying duties. Her third trimester must've been been pretty unproductive; imagine attempting Buffy-style gymnastics at a time when bending over, walking around or even prolonged sitting, are really quite uncomfortable.
I bet being a Slayer would have helped immensely during Nicky's childbirth though. Slayers heal quickly, and probably don't hurt as easily as normal people. Perhaps she would have had to be really careful pushing little Robin out though, with her superhuman strength and all. We wouldn't want her baby scooting out so fast he ends up shooting out the door of the delivery suite.
However, I doubt that the effects of sleep deprivation during Robin's babyhood, could have been avoided - even by someone with Slayer power. Perhaps that's why Spike was able to kill her in the end - the stresses and strains of early motherhood compromised the Chosen One's ability to keep on top of her game.
Maybe this is why only one Slayer ever managed to produce offspring.