[Not_a_pizza]

There is some psychology to this. I mean someone that is already healthy is more likely to do a greater amount of sexual things. If you're sick you're not going to be as sexual. Also, being sickly more common or unwell should already put you on unsteady ground when carrying a baby.

Additionally, a woman who has had more intimate exposure to her husband is probably less likely to have negative reactions to a fetus which they have created, judging from basic adaptive immunology.