How to Tell if a Fish Is Pregnant

For those who enjoy fish keeping, there’s nothing more peaceful and calming than watching their finny friends healthy and happy.  An aquarium is its own quiet world, and we can learn a lot about our fishes’ personalities and behaviors just by paying attention.  Maybe you’ve noticed something recently that’s piqued your interest – a bulgy, fish belly, perhaps?  How to tell if a fish is pregnant is not the biggest mystery in fish keeping.  We’re going to give you all the info you need to figure out, if indeed, any tiny newborn baby fish are on the way and what to do if and when those little fry appear.   

How Are Fish Born?

There are two ways fishes can be born, and the difference depends on the species.

‘Oviparous’ animals, which includes most species of fish, are egg-bearing. In this case, the female fish lays eggs, and a male fertilizes them outside the female’s body.  To determine if babies are on the way, you would look for mounds of tiny, jelly-like eggs near the surface or in protected areas of the tank.  These types of fish are not ever really considered “pregnant” because they do not carry the fertilized eggs inside their bodies.

Then, there are the ‘viviparous’ fish – those fish who give live birth.  Aptly known as ‘livebearers,’ they are primarily found among the Poeciliidae family, including aquarium favorites such as guppies, mollies, platies, and swordtails.  With these types of fish, the male fertilizes the eggs while still inside the female, and the eggs remain there until the end of the gestation period.

So, if you’ve ever wondered, “do guppies lay eggs?” now you know the answer is no.  Guppy eggs, along with all livebearer eggs, remain inside the mother until it is time to give birth; and when that big moment comes, a school of live, wriggly, newborn guppies will suddenly inhabit your tank! 

How to tell if a fish is pregnant will depend upon whether your fish is of the egg-laying or livebearing variety.    You may notice a small belly protrusion on a fish who is getting ready to lay eggs, but it won’t be there for very long; however, with livebearer fish, that belly bulge will get bigger and more pronounced over time. 

What Does a Pregnant Fish Look Like?

There are distinct physical signs of a pregnant livebearer fish, including the tell-tale belly bulge.  Usually located towards the back of the female’s abdomen, the swelling will take on a rounded or “boxy” shape and become more evident as the pregnancy progresses.  How long fish are pregnant depends on the species, but the gestation period typically lasts anywhere between 20 and 40 days.    

It’s important to note that a swollen belly in fish doesn’t always mean pregnancy.  It could indicate infection or disease, or even a bit too much enthusiasm at feeding time.  Fish can get big bellies if they are overeating – just reduce the amount of fish food and see what happens.  If the bulge gets smaller, you know your little glutton was eating too much; if your fish is pregnant, the bulge will remain and continue to grow. 

Also read – Don’t Buy a Guppy Unless You Read This

How to tell if a fish is pregnant is also easy if you look for a red or black spot near the rear anal vent; this is known as the “gravid spot.” It will get increasingly bright or dark as the pregnancy continues.  In the case of mollies, the gravid spot will become quite black as the translucence of the mother’s skin reveals the large, black eyes of the unborn baby fish!

How Do Fishes Get Pregnant, Anyway?

The ‘birds and the bees’ of fishdom involve a male and female, as is the case in most of the animal world.  It would be helpful to learn how to tell them apart, though it’s not always easy to do so.

Male fish tend to be more brightly colored than females; in livebearer fish species, the males are also smaller than females.  They have a long, skinny, rod-shaped fin on their underside, whereas females have a flat, fan-shaped anal fin.  

Being able to identify sex of the fish will help you tune in to any mating rituals going on, which can sometimes be mistaken for aggression.  If you see a male chasing a female around the tank, it could mean love is in the air (or in the bubbles, as the case may be).  Some species seem to hug each other and roll about as insemination takes place within the female.  If your fish lays eggs, then the male will release semen directly onto the eggs to fertilize. 

What’s that, you say?  You are quite sure there are only females in your tank, and they’ve been there for months – yet one is clearly pregnant!  Is it a miracle?  No, divine intervention has nothing to do with it.  Females can store sperm in their bodies for up to four months before using it to fertilize their eggs.  Guppies can actually use one deposit of sperm for 8 months and give birth every 30 days!  Interesting how one long-ago encounter with an ardent male could produce baby fish fry for months to come.  How fishes get pregnant can be a little surprising!

How to Tell if a Fish is Pregnant by Behavior

Pregnant livebearer females will start to behave a bit oddly as the time for delivery draws near.

You might think her restless as she explores the tank for the ideal spot to give birth.  Called “nesting” behavior, the mommy-to-be will swim around every nook and cranny until she finds a comfortable, protected spot to have her babies (aka “fry”). 

If your fish is very close to giving birth, you might notice her rubbing her rotund belly along the bottom of the tank or back and forth along the top of a leaf.  She might also become quiet and still for hours at a time as the weight of the growing eggs makes movement increasingly arduous.

Finally, if your fish completely stops eating, get ready!   Refusing food at her usual feeding time is a strong indicator that the fry are about to make their appearance.

Also read – Best Aquarium Lights for a Balanced Fish Tank

What to Do When Fish Have Babies

Whether your fish-babies emerge from eggs or are the result of a live birth, you’ll need to protect them from their parents and any other adult fish in the tank.  If you do not take action, the babies will likely become fish food! Livebearer fry has a better chance of survival because they are larger than the babies born from eggs. Still, all fry are vulnerable to predatory behaviors for a few weeks, at least until they are about 1 inch long.  

The best approach is to provide a separate 5-20 gallon tank for either the adults or the babies.  Ideally, you would recognize that your fish is pregnant ahead of time and move her into a separate tank before giving birth. After the delivery, you can put the mom fish back in the original tank.

Whatever you do, the important thing is to keep them apart.  If you choose to move the babies, use a cup instead of a net to remove them.  It’s important to use water from the existing tank to help the fry adjust to their new environment.  Also, keep in mind that babies can get sucked into a regular filter, so using a sponge filter is recommended.

Newborn fish have their own food requirements, as well.  Using liquid baby fish food, brine shrimp, or flake food ground into a fine powder are some effective ways of feeding fish fry and keeping them healthy. 

 It is highly advisable to speak with an aquarium store or vet to determine how best to care for your new fry, because different species will have different needs.  Consider it a challenge, and do your research.  Breeding fish is an exciting aspect of the fishkeeping hobby!  

How to tell a fish is pregnant – FAQs

How many babies do fish have? A typical livebearer school of fish fry can be anywhere from 5-20 babies, but it does vary by species. For example, guppies and swordtails can have anywhere from 20-100 babies per pregnancy and mollies between 20 – 60. While a goldfish can have anywhere around 45 -50.  That’s a lot of babies!

How long are fish pregnant? Gestational periods, as with most fish characteristics, will differ.  As a general rule, pregnancy lasts 20-40 days but can be longer.   For example, mollies can stay pregnant for up to 10 weeks before giving birth. The gestation period in aquarium fish is intricately linked to the water temperature, feeding, stress, and other tank mates. Maintaining healthy water parameters, using high-quality food, and keeping peaceful fish tank mates can ensure a normal gestation period. If any of the above factors are compromised, the gestation period will increase.

How long do babies have to stay in a separate tank? As noted earlier, fry are in danger of being eaten by larger fish until they reach at least 1 inch long.  It takes approximately 4-6 weeks for the babies to reach a safe size.  Maintaining optimal water parameters in the tank containing fish fry is crucial for ideal fish breeding. The safest approach is to use the same water from the original tank for the new breeding tank.

What do I feed a pregnant fish? Do not vary your feeding routine because of pregnancy.  Livebearer fish need both “algae-based and meaty” foods.  We strongly recommend flake food, freeze-dried bloodworms, and brine shrimp for a well-balanced fish diet, pregnant or not. It is best to feed amount small amount of food at regular intervals. Using high quality commercial fish food and frozen or live food will ensure that the pregnant fish and the fry remain healthy.

How do I know if my goldfish is pregnant? Goldfish are egg-layers and, therefore, are never actually pregnant.  They will become fatter, however, as the time to deposit eggs nears. Males will chase the female goldfish around to try and get her to release the eggs, and he will fertilize them outside her body.   Fertilized eggs will hatch in 2 or 3 days. 


How to tell if a fish is pregnant? is easy if you know what to look for.  But don’t forget to research the particular species in your tank.  Aquarium stores can be a wonderful resource for guidance and information.  Knowing the specifics about the kinds of fish in your tank will help you better care for them and appreciate their uniqueness for years to come!

Also read – Create Harmony and Diversity in Your Aquarium With the Best Community Tank Fish

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