My first and second baby took two days, the third took one day. The first and second babies I spent a lot of time with. The third I spent a good amount of time with. That baby was born from a npc though. Hopefully that helps
My kids won’t grow up and they are still newborns. I’ve had them for at least a month and a half. I’ve done the “Bundle of Joy” event about 15 times. ( for both ) . I don’t think it’s a bug..