Honestly we would not swing with a pregnant couple. But that's just us. For us, pregnancy is a very special "couple" time. We're not turned-off by pregnant women at all, though. They are sexy! Mrs. WS has said time and again she has never been hit on more by men then when she was pregnant.
Now the only thing we haven't considered, and it hasn't ever come around, is if it was a couple we had been playing with prior to pregnancy and they still wanted to play. That may be different.
A couple we know well did get pregnant, but playing during that time never came into the picture. If it ever does with someone I'll update this post.
Mr. WS