I remember about a couple ago about every 4 or 5 SGs I did was lactating. I myself don't like it as it sometimes get messy because as soon as you have sex with them there breasts start dripping like a faucet, well maybe that a faucet, but the beast milk starts coming out. I guess all the pounding somehow stimulates the breast milk to start pouring out, and usually the girl gets embarrassed and doesn't know how to stop it,and I don't want the milk as it belongs to the baby.
I haven't run into a SG lactating lately, but girls like that are fairly common in the zona, especially with SGs.