Message is quite vague, isn't it?
Copypasta to the rescue:
The "(You feel like something is preventing (???) from getting pregnant!)" message happens because of a multitude of reasons.
* She doesn't love you enough (has to be at love 4)
* She's already pregnant (wait a week)
* She has a kid in the daycare (can't have more than one)
* You have not been to the nexus (tutorial girl hasn't told you the rules)