Platy's get pregnant very easily, so more than likely yes your platy is pregnant. Females have more rounded off bottom fins and males have thinner, longer bottom fins. Generally you will need 2 females for every 1 male. When your female is ready to give birth it is better to seperate her in a mesh breeding net unless your tank is heavily planted. If you choose to separate the fry, you can feed them a variety of food. My favorite is yolk from a slightly boiled chicken egg mixed with water and fed to this fry from a dropper. Best of luck!