Pregnancy mechanics or just endings?
Played this game a long time ago.
The simple awnser to your question is that it's in the ending.
Long awnser and mechanics explained:
Basically during the sex mini game you have to match certain peices to have the main characters cum inside her.
(This is made easy by, sex expereince, equiped charms, and locking those specific matching peices on the board)
It's then left up to chance, (or again depending on what charms you have equiped) to succesfully impregnate her.
If successful you'll be able to see a little text in her status saying she's pregnant.
Finally the main diffrence is the last CG scene where she's either normal or pregnant with the MC's child.
Post game bonus: Depending on Haru's stats when she became pregnant, her stats will reflect onto the child where you can then go into some sort of mini dungeon with them as adults. I mostly stopped playing after the main story was finished so don't have much to say about the mini game other then I know there's more h-scenes with other npcs.