I've never enjoyed a pregnant SP, but I have enjoyed some pregnant women. There's things they don't tell you about in books, the unspoken truth! One of those things is: pregnant women are extremely and easily aroused, I think it's the pressure on their organs. Their pussy gets really hot and tight, nipples more erect than usual. Actually, I'd say the pussy is somewhat swollen, ymmv. If you can find a nursing mother, you're in for another treat! Nipples become desensitized and you can reef on them like pulling reins on a horse, no problem. There's other stuff too, let's just say don't eat or drink too much before the session!
You just have to get past the "baby in the belly" thing. Doggie style or spooning will be the only position feasible, but you'll love it.
I once found a pregnant woman on Quest (telephone dating line where I live)...they're out there.....hunt hard.