This Guppy is less than 12 hours from dropping her Fry. Guppies can have between 1 and 200 Fry and have a gestation period of between 22 and 30 days. Interestingly they can elect to halt/stop the pregnancy. They can also give birth up to 6 times from the same mating. That's why if you are serious about breeding you need to separate your female fry as early as possible so that you can control who she mates with, that's the only way you know who the father is for sure!
One of the most common questions I get from newbies is how to tell when a Guppy is going to drop her Fry.
It's pretty obvious once you have had some experience although with all the telltale signs it's still very possible to get it wrong. After all, nature tends to do it's own thing more often than not!
There are some really good clues though. The picture of this female guppy was taken less than 12 hours prior to her dropping about 25 Fry and is at a good angle for trying to describe the "signs".
I tend to separate my females when I notice the tummy starting to square off, usually about one week prior to her giving birth. This gives her time to settle, minimises the stress from the male constantly harassing her. I also use breeder boxes that hang on the outside of the tank. She can still see into the tank, the other fish can see her I don't know because I'm not a Guppy mind reader but it seems to reduce the stresses of being separated.