I have been reading about pregnant guppies and have seen many times people saying that a water change will prompt them into giving birth soon afterwards.
Sorry, I don't know, but I do know that you need frequent water changes for fry. You can tell when a guppy's about to drop when has a square belly with a white dot near her bottom fins. Their pregancy usually lasts 28-30 days, but don't be surprised if you have to wait a month and a half between two drops! I don't see anything wrong with this theory, however, so I think it could work.
Yeah, I'm waiting for her to drop again. She looks so big. I have a photo of her the night before she dropped her first ones and I'm comparing her to that now and she looks almost the same. Today is 30 days since she last dropped.
It sounds like she is about to go again Joanne_.
I have never seen a water change induce a drop but anything is possible I suppose. On the other hand, you do want to keep your water clean, especially when there are fry coming.