Pregnant Platy?
 

I have a tank with 3 female platies and 1 male. I recently got back from 5 months of working up north and my dad took care of my tank while I was up there. He said one day he noticed a bunch of little baby fish in the tank but wasn't sure which fish had babies (I guess he didn't pay attention to them much). They were all eaten up by the other fish in the tank. However, one of the plates has a very large stomach. I was home about a month ago and she looked pregnant back then too.
My question is, what is their gestation period and how can I know when she is going to give birth? I have a breeding net to use but I don't want to put her in there if I don't know for sure she is going to have her babies.

A picture would help immensely, but with the information provided she probably could be due any day now. There gestation period- 4 to 7 weeks normally more around the 4 week mark. To tell when she is about to give birth the black spot by her anal fin will become very dark (Babies inside her) and she will seem square.