Oh goodness I am in the same situation! I have a pregnant platy also! I've been breeding tropical fish for about a year now and I'd love to give you some advice! One of my mickey mouse platys is pregnant also. What I do is I check up on my tank a lot. When I see fry in my large tank I put them in a breeding net. You can get breeding nets at almost any petstore. I highly recommend the one from TOP FIN because it is very cheap, and it works amazingly! I've had it for a year and it's never doubted me! I have that in the corner of my tank and as my fry are swimming around, to prevent them from getting eaten, I use my net and catch them and put them in the nursery. I don't really recommend the plastic breeders with the cracks at the bottom because of the lack of air flow and I think they would most likely get eaten in those by the mom if you don't take them out in time, plus being in the nursery can stress out the mom. Also keep in mind when you catch the fry in the large tank if that is what you decide to do, babies have a natural instinct of hiding, so some of them might be hiding in the gravel. Keep the net in hand just in case you just so happen to see a little fry peering out of the gravel or plants.
Hope this helps!
Momma's chillin in the tank with the kidsss.
Sushi the HALFMOON