Several thoughts come to my mind: Maybe he is mentally deficient. Maybe their relationship was on the rocks and he didn't see her much. Maybe he was in on it.
I keep wondering how this woman could do this alone--I guess she could have, but it sure seems like there might have been some help.
I haven't seen any picture of the supposed pregnant woman, but in the Ohio case she was fat (excuse me, "large") and her husband believed her when she said she was pregnant.
http://archives.cnn.com/2000/US/10/05/pregnantwoman.killing.ap/
In fact, a number of fat women have supposedly given birth who didn't even know they were pregnant. I find that hard to believe unless you are a bit slow.