Just make them all ineligible to randomly become pregnant until the quest is completed.Some of it is partly due to a debate about how to handle the dance girls getting pregnant, since if any of them are "too big" according to the dialogue there, theoretically the entire storyline is put on hold, possibly ending up with a softlock of the story where at least one girl is always disqualified due to pregnancy.
Solution so obvious, I don't understand why it's not already implemented.