Mollies can store sperm in their bodies allowing them to have multiple pregnancy's after only mating onceThe cool thing about Platy is once they are pregnant, they seem to give birth multiple times before getting pregnant again (I don't know any exact details such as how long in between spawns and how long the gestational period is) but I do have a tank of 3 platy (2 girls, 1 boy) and a male betta (it's an NPT as well) and I have one "juvie" platy, and a ton of infant platy, and the girlies are still pregnant. It's a lot of fun. If you let them give birth in tank that is not set up for it, a most of them won't survive simply because they will be eaten (which isn't necessarily bad if you don't want more). However, I did have an experience in my 55 gallon tank in which I didn't even know any of the fish were pregnant and one day I see two juvies swimming around. They are now full adults and have grown up in that same tank, which is kind of cool.