When Mina (my female deceased guppy) had her babies, I had to move her to a smaller, separate tank from her husband, Zack. She took a good week to a week and a half or so before she popped those little suckers out, and the only way we knew it was because Zack was having a fit over at the side of his tank that faced her. I don't know about platys, but guppies can hold it off a little while, for some reason.