I agree with Innes, you need to seperate the mother Guppy or Platy and then when she's done birthing seperate the mother from the babies. If you have a small tank, like a 5 or 10 gallon full of plants this works great as a birthing tank and transition to a grow out tank. It also doesn't stress the mother as much as a net does. When you first put the mother in a net she will act totally stressed out. Plants and having the lights off will help her to adjust to being in the net. Good luck, hope it goes well for you.
mak