I believe the platy and sword tail females can store sperm for a long time, so that the male in your tank is not necessarily the father of the fry you are expecting. I think they can also have multiple batches of fry from stored sperm, so even future fry might not have any relation to the male in your tank. Isolate the pregnant female in a tank that also has fry cover, or add fry cover to the tank she is in now. One day you will notice the funny little things hovering around the fry cover, and a few of them might even be quick enough and smart enough to grow up.
I had a Platy that I just thought was well fed, didn't notice the fry till it was too late. Now I keep some fry cover in the tank for next time.