Im my particular experience it would be harder to keep the mollies from getting pregnant than worrying that they will not get pregnant. They are so cute when they are so big that they look like they are going to explode. Keep lots of plants (fake or real) floating at the top so that they can hide from the other fish. If they are white fry they are difficult to see at first because they are almost completely translucent. Other than that they pretty much take care of themselves.