How do you care pregnant molly fish?

Exactly the same way you look after any other livebearing fish. 1 inch of fish needs a minimum of 1 gallon of water, a cycled and running filter, and a minimum of 50% of the water changed every week.

You don't need to treat her any differently because she is pregnant, provided that your normal tank conditions are ideal. Towards the end of her pregnancy she will eat a little more than normal, but not enough so that you would need to noticeably increase the amount of food you put in the tank.