I think there was a specific message that popped up right before triggering the pregnancy event with Bianca.
All the characters have a "fertility" value within the save file.... I tested if the message about only once and only one character was true by changing the "fertility" values in one of my saves... I gave different values to each character the highest I gave to the Mayor at 400 and she still mocks you after cow girl sex with a comment about she might get pregnant in the future but today she's not very fertile.... currently the "fertility" value never changes so the pregnancy system is definitely not active yet.... I am looking forward to some bouncing bote ("Bote" is basically "Baby Bump" in Japanese... it will get responses on you website of preference for waifu enjoyment.)