How to tell if a Molly is pregnant

If you have fish, you might have wondered at some point whether or not they could get pregnant. And if you’ve wondered that, you might have also asked yourself if you’d be able to tell if they’re pregnant.

It’s good to be able to know the basics and to be able to recognize the signs of a pregnant molly fish, that way you’re aware of the situation before your tank is suddenly full of fish babies swimming around! 

Molly fish, better known as simply “mollies”, are freshwater fish that are indigenous to the Southern part of the USA and Mexico. They are very easy to take care of and therefore a very popular home pet!

What’s curious about the Molly fish, is that it’s a livebearer. This means that it retains the eggs inside the body until they hatch, and then gives birth to the live fish!

So unless you can tell when your molly is pregnant, you won’t even be aware of the babies until they come out already swimming, no visibly laid eggs to warn you!

Your molly can get pregnant very easily, as they are very prone to breeding if there are both male and female fish in the tank. (That should probably be something that you’re aware of, so you know whether to bother looking out for pregnancy signs or not!)

It’s also very important to be aware if one of your mollies is pregnant, because as soon as it gives birth, there will be little molly fish babies swimming around the tank, and that can be a problem.

Not because your tank will be overpopulated or anything, but because the babies might get eaten by the other fish in the tank! They might even be eaten by other mollies, so if you want to avoid fish cannibalism, you need to know the tell-tale signs of pregnancy. 

Steps to follow to check if your molly fish is pregnant:

Molly fish have a naturally rounded and “chubby” belly bottom, so it can be hard to figure out whether or not they’re pregnant.

And unless you somewhere speak fish, you can’t exactly ask whether it’s expecting or not. In order to check for pregnancy in your molly fish, there are a few steps you can follow, which if done carefully, should always allow you to know for sure. 

Step 1

The first step to figuring out whether your molly fish is pregnant is to observe her behavior.

You’re going to have to pay attention each time you check on your aquarium and be able to easily single her out for observation. (This can be tricky if you have several and identical-looking molly fish, but hopefully, you know your own pets well enough to single her out!) 

Behavioral changes that might indicate a pregnancy might include moving slower, so check if her swimming patterns are slower and less energetic, as that might indicate that she’s tired and heavy with babies!

It could also include changes such as eating more (because she’s now eating for many!) or staying away from other fish. 

However, these behavioral changes don’t always signify a pregnancy, so they’re a stepping stone to move on to the second step. 

Step 2

For the second step, you’re going to have to take an even closer look at your molly fish. If it moves around a lot or hides in corners with low visibility within the aquarium, then you might struggle.

If it comes to it, you might need to carefully handle the molly fish and put it somewhere smaller so that you can observe a lot easier. 

What you need to look out for and try to find, is a dark triangular spot near the anal vent. If the molly fish has this, then it means there is a pregnancy, and babies on the way. 

If your molly fish is orange or white, this should be pretty easy to spot. However, it might be a lot harder to notice on a black colored molly fish, so if that’s the case, you’ll have to look even closer and try to find a darker tone of black in the triangle-shaped spot. 

Step 3

Once you’ve spotted the triangle mark near the anal vent, you need to keep the molly in observation to see how the situation develops. With a pregnancy, you’ll be able to see how the molly fish’ stomach begins to change.

It will grow in size and stretch outwards until it is pretty visible and makes your fish look fat. This is a huge and certain sign of pregnancy.

Step 4

Continue your observations of the molly fish. As you might have noted, keeping up with your fish’s pregnancy will involve a lot of staring into your aquarium, but it means you’ll learn a lot about it!

As the pregnancy advances, the stomach will stretch even more. It will reach a point in which you can detect the eyes of the fry in the molly fish’s stomach. It will also keep growing bigger in size, which is completely normal during pregnancy and is nothing to worry about.

The behavioral changes you had to look out for in step one will also be a lot more noticeable by now. The molly fish will swim extremely slow, tired, and heavy. It will stay away from other fish as an instinctive reaction to the situation, and it will be eating a lot more than on average. 

If you observe closely, you will also notice that the dark triangle-shaped spot near the anal vent gets bigger, becoming larger and darker the closer the molly fish is to giving birth. 

A molly fish pregnancy lasts between 20 and 40 days, so that is the amount of time during which you will have to monitor the situation, and it gives you a rough estimate as to when to expect the birth. 

When a molly fish gives birth, as mentioned at the beginning of the article, the babies are already alive and swimming, as the eggs have been retained inside the body. 

A molly fish can give birth to a number of babies between 20 and 100, at a time, so there’s a big range of possibilities! 


Molly fish are livebearers, meaning they retain the eggs within the body and give birth to live baby fish. 

The pregnancy lasts between 20 to 40 days, and the molly fish can give birth to a number of babies between 20 to 100.

The first thing you will notice are behavioral changes such as swimming slower and eating more. Then you’ll be able to spot a dark triangle near the anal vent. The stomach will grow and stretch, as the dark spot grows larger, and the behavioral changes will be enhanced. 

It is very important that you take out the babies when they are born, as they will otherwise be eaten by other fish in the tank.