as a general rule niche always converts better
I would go for recurring sponsors as any one with a fetish for pregnant women is much more likely to be a member for a longer period of time.
Adult Paymaster ,
Pimp Roll &
Silver Cash all have good converting, well designed preggo sites with both per signup and recurring payment options and all pay with epassporte.
also...
my friend runs a program
http://www.exclusively4affiliates.com - an amatuer/milf type program. One of the girls in his network of paysites is his wife who is pregnant at the moment so they are taking plenty of pics for wm's and pushing that angle hard for the members that like it
Just a thought. You can contact him thru icq 91-965-885 if you wish
all the best