Are my platties pregnant?

Question
I have a large assortment of fish. Recently I purchased four patties strictly breeding. I haven't received my breeding tank yet; although, it should arrive in a day or two. The patties are now located in a separate tank that I set aside for situations like these. The male platy continues to chase a female around the tank. Are they trying to mate? Also I have one platy that isn't very active. She just hides in the plants or stays close to the bottom. Is it a chance that she is pregnant?

Answer
The male and female could very well be trying to mate. The one that is sitting off to herself though... what color is she? Often times with livebearers you can actually see the babies eyes inside her. They are located towards the tail, before her vent. If you can't see the eyes, it sounds more like she's stressed out. Maybe from all the traveling before she got to your tank? How long have you had her?