I only ever seem to catch them in the daytime Jen. It is when I am up. I really don't know when they "usually" give birth because I will see them very pregnant one day and by the time I come home from work the next day there are fry in the tank. I only really see the births when it happens on my day off work. There is a female molly in one of my tanks that I may catch this weekend though. She is definitely looking ready to start and is now hiding from her fry in the birth tank. Her fry are about 4 weeks old.