I don't have the actual studies handy, but I can tell you that there is evidence that suggests that previous cesarean can lead to a few problems with conception and sustaining a pregnancy such as:

 

- increased risk of ectopic pregnancy;

- increase in miscarriage;

- increase in placenta previa and placenta accreta. 

Every cesarean a woman has increases her risk of experiencing these issues.

I can't explain the ectopic pregnancy, but one theory I have heard about miscarriage, previa and accreta have to do with the scar tissue and where the placenta grows. 

Some women also have a more difficult time with sex after a cesarean.  Some women simply don't want to do it, some women will participate but have a hard time during the act.  I read a study a while back that suggested that the chemical reactions our body goes through during orgasm aid in conception.

-R