Well if she has been getting bigger for a few weeks, then she is probably ready to give birth any time now. Just keep an eye on her, sometimes they act different when they are ready to give birth. I have had people say that the wont eat as much, or they stay away from other fish. My guppies didn't stop eating before birth, infact one of my guppies who gave birth to about 65 fry had a very long labor, over 24 hours, and I fed her a little in her tank, and she ate durning labor. She also looked like she was going to "burst" for about a week before she gave birth. You just need to watch her behavior....is she in a tank with other fish? If she is, and you want to keep them, I would suggest getting a breeders net, or tank. I have a breeders trap that I have for my mothers to be. You just put them in there when you think they are ready to give birth, and it saves almost all the fry, some DO find there way back up, but losing a few and still having over 50 is pretty well i think. If you don't want to keep the fry, the other fish will eat them in the tank, and the mother will too! You can also try breeding grass. Or have lots of places for the fry to hide. Good luck!