How to Tell if Swordtail Fish is Pregnant?

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Swordtails are extremely popular as aquarium fish thanks to their easy-going attitude, adaptability, and long lifespan (4-5 years in good conditions).

They belong to the Poeciliidae family, which makes them related to guppies, platies, and mollies and provides them with similar biology.

If you know how to care for guppies, you know how to care for the swordtail. Their name stems from the male’s prolonged caudal fin, resembling a sharp sword.

The females don’t have that, making it a lot easier to differentiate between the sexes.

Today, we will discuss swordtail pregnancy signs. How can you tell when the female swordtail is pregnant and about to give birth, and what should you do?

Swordtail Fish Pregnant Signs

Like all Poeciliidae members, swordtails are rather proficient breeders. They can easily breed in captivity and produce a lot of offspring every month.

With proper care, the fry will grow fast and adapt to their environment in no time.

But before going there, you must first learn the key signs showing that your swordtail female is pregnant.

These include:

Growing Belly

This is a common pregnancy sign in all living creatures, fish included.

The female will display no obvious sign at first, but everything will become clearer 2 weeks into the pregnancy.

The belly will grow visibly, sometimes at an accelerated pace as the delivery moment approaches.

If you’ve excluded overfeeding and disease as potential causes for this physical change, pregnancy is the only other option.

By the time you will notice this change, you will probably be left with around 2 weeks to prepare for the coming fry.

I suggest considering a breeding tank to keep the female safe and prevent fish stress. It will also help you monitor the female’s progress throughout the pregnancy to make sure everything goes smooth.

As a plus, the female’s belly will take on a more square shape by the 28th day of the pregnancy and look like it’s about to pop.

That’s the sign that the labor is just around the corner.

Gravid Spot

The gravid spot is easier to notice in some fish than others. This is a dark area in the fish’s abdomen, towards the cloaca, visible from both sides of the body.

The gravid spot indicates the formation of eggs and, later, the fry developing inside their mothers’ wombs.

Swordtails, like all fish, reproduce via eggs. However, livebearers, like those in the Poeciliidae family, have their eggs hatching inside their bodies.

As a result, they will give birth to live fry.

These are visible in the female’s abdomen towards the end of the pregnancy.

At that point, the gravid spot will grow larger and become darker as the fry grow and take up more space.

The dark color is nothing more than the fry’s eyes crowded in the same spot.

Increased Appetite

Swordtails usually eat around once or twice per day, depending on how many of them you have.

If you have a larger fish tank and a lot of fish, not all of them will get access to as much food as others. This means you need to feed the fish at least twice a day to make sure everyone gets their fair share.

Swordtails aren’t generally big eaters, but that changes with pregnant females.

They will eat more food, more frequently, and will fight for it. Food-related violence isn’t uncommon among fish, especially when it comes to fry-bearing females with more on the line.

If you’ve already determined that your swordtail is pregnant, expect to require more food than usual. After all, it needs the extra calories and nutrients to support its developing pregnancy and grow the fry.

Hiding from Males

Swordtail males are always competitive when it comes to their mating rights.

Male-on-male swordtail violence is common in populations with too many males and not enough females. This is why it’s always a good idea to keep at least 2-3 females for every male in the tank.

But that will only fix one part of the problem. The other part involves the males’ pushy behavior and desire to pass on their genes by any means necessary.

Males don’t care if the females have already mated and are no longer interested in their services.

They will keep advertising themselves by pursuing the female around the tank, poking it, and testing their luck.

This behavior can stress out the female and force it into hiding. This is not a healthy environment for your pregnant female swordtail since stress can lead to a variety of health problems, including a weaker immune system and even premature birth.

In the case of the latter, many of the fry won’t survive.

If you notice that your female swordtail is constantly chased by males around the tank and tends to hide, she might be pregnant.

In this case, if the males’ persistence doesn’t wear off, consider moving the female into a different habitat, preferably alone, until the pregnancy reaches term.


Swordtails are generally easy-going fish, only displaying necessary violence when it comes to enforcing territorial rules and specific social hierarchies.

They tend to be mostly peaceful, minding their own business and avoiding fighting whenever possible.

That changes a bit during pregnancy when females become less tolerant of their tank mates. Pregnant females may snap at anyone in their vicinity.

They won’t necessarily resort to excessive violence, but you should be able to see a lot of poking and chasing.

This is an indication that the female’s behavior has changed, and you should look for potential explanations.

If the female is well-fed, healthy, and active, pregnancy is most likely the culprit.

Not all females display the same behavior or the same intensity, but many will. If they tend to become over-aggressive, you should consider separating them from the bunch.

The first 2 are the most relevant and easy to observe out of all these signs. Swordtails don’t usually grow their bellies without a good reason.

If it’s a female, that reason is most likely pregnancy.

How Long are Swordtails Pregnant?

The typical timeframe for swordtail pregnancy is around 28 days. It may last longer than that in some cases, but not by much.

As a general rule, if the pregnancy period extends for more than a month, something is not quite right.

Stress is a common pregnancy delaying factor, especially if you have a crowded tank with several fish species.

You should monitor your female swordtail closely after its pregnancy becomes visible. If you see her hiding more than usual, despite not being close to delivery, she may be experiencing stress.

A good way to minimize fish stress is by decorating the tank with a lot of plants and hiding spots.

This allows the fish to retreat to its safe place to escape bullying and avoid situations it doesn’t feel comfortable in.

How do I Know When My Swordtail Fish Will Give Birth?

There are several signs announcing an upcoming delivery.

These include:

  • Loss of appetite – The female swordtail will eat more than usual throughout its pregnancy but stops eating completely when labor approaches. If your pregnant female swordtail refuses food completely, that’s a sign that labor will set in shortly.
  • Hiding behavior – If your pregnant female tends to hide near the substrate for no reason, you should consider her time is up. Pregnant swordtails tend to look for hiding spots instinctively around the 28-day mark, and you will see them swimming near the substrate and displaying low levels of energy.
  • Changes in belly shape – The belly will slowly turn slightly square in appearance rather than round. It will also appear like it’s about to burst, allowing you to see the uterus pressing against the female’s stomach lining. At this point, the female swordtail is hours away from delivering the fry.
  • Visible contractions – Fish experience birth contractions, too, and they look similar to those experienced by most animals. The swordtail will shake its body and fluster and shiver as the labor sets in. The entire process may last for hours, depending on how young the female is and whether she’s had any other pregnancies before.

All pregnant swordtails will display these signs when nearing the birth moment. Identifying them in time allows you to control each situation more effectively.

How Many Babies do Swordtail Fish Have?

Swordtails will usually produce around 30 to 35 fish per month, every month.

However, the number of offspring will depend on a variety of factors, including the female’s age.

Younger female swordtails may produce as little as 5-8 fry during their first pregnancy. The older and more experienced the female is, the more fry it will produce.

The delivery time also becomes shorter in older females, whereas younger ones will take hours to give birth to all their fry.

Expect your swordtail to produce as many as 80 to 100 fry as they grow older and become more proficient breeders.


Swordtails breed every 30 days roughly, just like all members of the Poeciliidae family.

This makes them quite effective breeders, capable of growing their population fast.

However, some issues are worth mentioning here.

If you want to save as many swordtail fry as possible, you need to set up a breeding tank for several reasons, like:

  • Protecting the female – Female swordtails can become cranky when the pregnancy nears its term. You want to prevent your female from experiencing stress and discomfort during labor to ensure smooth fry delivery. Moving the female into a breeding setting when labor draws close will help you achieve that.
  • Protecting the fry – The fry’s instinctive prerogative is to survive. The adult fish’s prerogative is to eat them. These goals are clearly contradictory, and only one side will win. Adult fish will always hunt the fry for food, including their own parents. If you want to avoid that, a breeding tank is the best solution at hand.
  • Optimize your fry’s growth – Swordtail fry require slightly higher environmental temperatures and more protein-rich food. Provide them with a separate tank to grow healthy and comfortable in a personalized setting, and your fry will thrive.

As a closing, if you decide to keep the fry in the main tank since birth, provide them with a lot of surface plants.

Swordtail fry like to swim around the water’s surface, which will make them more visible to other fish.

Hornwort, java moss, water wisteria, ludwigia repens are all great examples of floating plants that will keep your fry safe and comfortable.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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