How Can You Tell If A Molly Fish Is Pregnant? (With Pictures)

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I remember how challenging it can be to identify pregnant molly fish. In some cases, the fish isn’t actually pregnant but sick. So, based on years of experience, I decided to gather all the signs you should look for to determine whether your molly fish is pregnant.

These signs indicate that a molly fish is pregnant:

  1. The molly fish’s belly will swell.
  2. The gravid spot will appear next to the anal fin.
  3. The fish will go into hiding.
  4. The appetite of a pregnant molly will increase.
  5. The fish will become aggressive towards tankmates.

As we move forward, I will elaborate on the early and late signs of a pregnant molly fish. Followed by pictures, this article will help you identify pregnant molly fish in different stages. I will also include a Youtube video that shows how a molly fish behaves when it’s about to give birth.

Also Read: Pregnant Molly Fish 101

How To Tell If Your Molly Fish Is Pregnant

Molly fish are livebearers. When the male fish inseminates the female fish, embryos will form in the fertilized egg cells, producing live fry that the female molly will push out later on once they develop fully.

The gestation period lasts roughly 70 days. Though, the female molly can fertilize its eggs every 30 days even if the tank doesn’t have a male fish. That is because female mollies can store sperm.[1] 

During the gestation period, the female molly will manifest the following signs:

Early Signs Of A Pregnant Molly Fish

  • The Molly Fish’s Belly Becomes Swollen

This is the most common sign. However, it is also the most complicated. The belly doesn’t swell immediately after the male molly fertilizes the female. You have to wait until the eggs become embryos, which can happen seven days after conception.

The belly size will depend on the number of fry the molly is carrying. As the fry develop, the molly’s belly will continue to grow. The creatures can accommodate as many as 120 babies at any given moment.[2]

This sign is complicated because the stomach of a molly fish can swell for other reasons besides pregnancy. The most problematic of these reasons is dropsy. It causes the belly to fill with fluids, damaging the organs as it swells.[3]

Other notable causes of bloating include stress, constipation, bacterial and parasitic infections, gas, etc. Diseases like dropsy attract additional symptoms, including bulging eyes and pale gills.

The same cannot be said for stress and some infections. Your best bet is to look for other signs of pregnancy, such as the gravid spot.

  • Your Molly Will Develop A Gravid Spot

You can use the gravid spot to track embryonic development in molly fish.[4] The spot’s size and intensity speak volumes about the development of the fry. If you’ve never seen one, the gravid spot is a dark triangular spot that neighbors the anal fin. 

The gravid spot marks where the fry inside the female molly have clustered. You can probably see the developing fry inside the fish’s body if you have a UV light. 

Because the gravid spot definitively proves that the fish is pregnant, people panic when they fail to see it. However, you must realize that the gravid spot doesn’t make the pregnancy happen. Even if the female molly doesn’t have a gravid spot, it will still give birth. 

The gravid spot is just a sign, not a tool or mechanism that the molly requires for a successful pregnancy. Additionally, because the gravid spot is dark, you are less likely to notice it on mollies with dark colors. This mark is easier to identify on lighter fish with bright, more vibrant colors.

Also Read: Where Is The Gravid Spot On A Molly Fish?

  • Your Molly Fish Will Go Into Hiding

Pregnant mollies can become a nuisance because they tend to develop aggressive mannerisms. Most pregnant mollies won’t attack their tankmates. They are more likely to go into hiding, especially if their tank has plenty of hiding places.

They may frequent areas within the vicinity of the heater and other heat sources (such as UV lights) because they want a stable and reliable source of warmth. This is the reaction aquarists want because it maintains peace.

However, some female mollies can become a nightmare, particularly in the final days of their pregnancy. The attitude you will get depends on the personality of the fish. Ultimately, pregnant mollies do not want other fish to bother them. 

If your fish are smart enough to leave the pregnant molly alone, the molly’s pregnancy won’t attract any violence. But if the pregnancy has ignited violent conflicts, you should consider moving the molly to a new tank, regardless of whether or not it sparked the disputes.

The Youtube video below illustrates a pregnant molly fish that will soon give birth. As you can see, the fish spends most of its time hiding. If your tank features plenty of vegetation, you are likely to find your pregnant molly hanging in that area:

  • The Molly’s Appetite Will Increase

Like humans, a pregnant molly’s appetite will increase because it has to meet the nutritional needs of the developing fry. Stressed mollies may avoid food during this period, but those cases are rare. A pregnant molly’s appetite usually spikes.

You can start looking for these signs seven days after fertilization. While mollies are expected to remain pregnant for an average of 70 days, that is an exaggerated estimate. The actual figure is closer to 20-40 days for many mollies.

But you don’t need that estimate to figure out when the molly fish’s pregnancy will end. If the fish is about to give birth, you will observe these signs:

Late Signs Of A Pregnant Molly Fish

  • The Gravid Spot Will Take On A Square Shape

During the early stages of pregnancy, many amateur aquarists cannot determine whether they can see the gravid spot or if it’s just a trick of the light. But that becomes less of an issue as the molly approaches the final stages of its pregnancy.

As the pregnancy progresses, the molly’s body will expand to accommodate the development of the fry, causing the gravid spot to become larger and darker. Additionally, this region will take on a square shape.[5] The fry are easier to see with the naked eye.

  • Your Pregnant Molly Will Become Aggressive

Rather than simply avoiding the attention of other fish, the pregnant molly will go into hiding because it wants to release the fry in a safe and peaceful environment. Pregnant fish are more likely to become violent in a tank with insufficient hiding places because they don’t feel safe.

If your tank has plenty of hiding places, the female fish is more likely to lash out at intruders as it nears its due date. 

However, don’t forget to look for other signs of trouble. Negative factors such as infections, stress, and bullying can also send a molly fish into hiding. 

If the pregnant molly has been in hiding for weeks, but you haven’t observed any new fry in the tank, the pregnant molly is hiding for other reasons, or it reabsorbed the fry.

In other words, it can refuse to give birth. This happens in poorly maintained aquariums that have created a stressful and unsafe environment for the female molly.

  • The Pregnant Molly Will Stop Eating

If the pregnant molly has stopped eating altogether, it is about to give birth. Again, make sure you have ruled out other factors that can extinguish a molly’s appetite, including infections. In this case, you’ll also notice lethargic swimming and color changes.

  • The Molly’s Belly Will Become Round

The belly will continue to swell until the area under the gills becomes round, showing that the womb is stretched to capacity. The fry will become even easier to observe.

At this stage, you must choose to either leave the pregnant molly in the tank or move it to a separate aquarium. Otherwise, it will eat the fry.

Many aquarists will encourage you to make that decision early on. The stress of moving to a new tank could harm female mollies in the final days or hours of their pregnancy. They may respond by reabsorbing the fry. You can also move the female fish after it gives birth. 

But that is only helpful if the female molly lives alone in an aquarium, in which case, removing the adult molly allows the fry to inhabit the aquarium alone. Allowing a female molly to give birth in a community tank leaves the fry at the mercy of the molly’s tankmates.

Is My Molly Fish Fat Or Pregnant?

As mentioned earlier, molly fish can be fat and bloated due to other reasons than pregnancy. That includes constipation, dropsy, swim bladder disease, etc. The signs above may indicate that your molly fish is pregnant.

However, the following may indicate that your molly is fat because it is actually sick:

  1. The molly swims sideways or vertically.
  2. Your molly features ripped fins.
  3. The aquarium doesn’t occupy male molly fish.
  4. The molly is staying at the bottom or the top of the tank.
  5. Your molly breaths heavily and rapidly.
  6. The molly changes its color (primarily to white and red).

If you notice the signs above, start by measuring the water parameters. You can easily do that with the API Water Test Kit (link to Amazon). As a rule of thumb, the ammonia and nitrites should be at 0 ppm. The pH should be between 7.5 and 8.5, and the nitrates below 20 ppm.

If the toxins are too high or the water is acidic, I suggest performing more frequent water changes. If not, consult aquatic veterinarians. They may diagnose an underlying disease and treat it with antibiotics.

Either way, when kept isolated, you may be surprised to find that your molly fish actually gave birth to living fry. That is how misleading a bloated molly fish can be. But if the fish is indeed sick, you’ve already isolated it and prevented the disease from spreading.

Also Read: Can Molly Fish Get Pregnant Alone?


You can tell that your molly fish is pregnant by its behavior and general appearance. Early signs of pregnancy include a swollen belly, a darkening gravid spot, and an increase in appetite. You’ll also find the fish hiding quite a lot.

Late signs of pregnancy include a square gravid spot, a stretched abdomen, and aggressive tendencies. If your molly fish is actually sick and not pregnant, you’ll see symptoms such as labored breathing, ripped fins, color changes, etc.