If you are planning to keep the fry that your female guppy fish produces, you will want to know approximately when she is due to drop her fry (have baby guppies). Adult guppies eat the baby fish, so it is imperative to isolate your pregnant female guppy when she gets close to the end of her pregnancy, or else the other guppy fish in the tank will probably eat them all.
Like most creatures, as a guppy’s pregnancy progresses she will gradually grow a round belly that gets bigger and bigger. When your pregnant guppy is getting close to her delivery date however, you will notice that usually her belly will square out a bit. That is to say, instead of just looking completely round, her belly will start to get a little flat on the bottom while still remaining rounded at the sides.
Another great way to approximate your fish’s due date is to note that a guppy pregnancy can last anywhere from 21 to 30 days, but the most common gestation period is 28 days. Thus, if you keep track of when your virgin female guppy was first mated to a male guppy, or you make a note of when the last birth occurred, you can expect her to give birth 21 to 30 (but most likely 28) days later. You can track important information like this in a little journal with a calendar.
Depending on how good your eyesight is, you may also be able to actually see the babies inside the gravid spot of certain colors of guppy. This is the technique I use most often when I have no idea when the female became pregnant and I am trying to guess when she is due. Blonde and lightly colored females are the easiest to see babies in. The darker females, such as gray and black can be quite difficult.
You can do this in two ways: Either watch and wait for the female to swim under the light at the right angle so that the light is behind her or place her in a little separate tank like a photography tank (with the same tank water of course) and hold the little tank up to or place in front of a light source. Either way, you will need to wait until she is sitting still or you will not be able to see anything. Once you get a good view, you will probably see little black or dark dots in her gravid spot if the is in the later part of her pregnancy. These little dots are actually the baby guppy fish’s eyes! The tiny eyes will be more easily visible and well- formed when they are close to being born. I have even seen the baby fish eyes move and look around through their mother’s gravid spot when she is very close to delivery! There are not many creatures with such a “window to the womb” naturally occurring, it is just one more reason that keeping guppies can be amazingly interesting.
Over time as you get more experience, you will get a sense for when pregnant female guppies are close to having fry. You will learn the gestation time for your individual strains and how individual guppy fish will look when they are close to giving birth. Use the tips I mentioned and observe your guppies closely and you will have a great head start at knowing how to tell when your pregnant guppy is going to have baby guppies.