They did not leave a lot of room for such surprise pregnancies.
I suppose Cathy Geary showing up with Ben's baby in S8 could have been a convenient excuse to have him leave (with her), if not just so we could avoid the Jean Hackney nonsense. I'm picturing an infant in a crib with spiky, platinum blond hair and a torn-up sweatshirt instead of a onesie.
It would have been really soapy if Diana realized she was pregnant shortly after Chip's death. Abby would advise her to get an abortion, and after she does Karen would find out and be angry....which would force Karen to re-examine her feminist bona fides while also re-igniting the Abby/Karen feud. It would also drive a wedge between her and Mack, who would actually support Diana's decision.