How To Tell If Your Fish Is Pregnant

Whilst the majority of female fish species lay eggs, there are a few specific breeds that give birth to live young, known as fry, which they carry to full term. Referred to as live-bearers, or (ovo)viviparous fish, they hold the egg inside until it’s hatched.

Such fish give birth after an average gestation period of four weeks, though this varies between species, and the easiest way to establish pregnancy is by observing for the presence of visual cues and ruling out all other possibilities, first. 

Which Fish Give Birth?

Of aquarium fish in captivity, livebearers typically belong to the categorization Poeciliiidae, which includes mollies, guppies, moonfish, platies, Endler’s livebearer, mosquitofish and swordtails.

Some are ovoviviparous, which basically means the egg is not fed by the parent, whereas others are viviparous, and are nourished by the maternal blood supply; both kinds of fry are larger and much easier to successfully raise than eggs that are laid.

Male Or Female

Before confirming whether or not your fish is pregnant, you should probably rule out if it’s a male or female, unless you already know for sure. There are a few signs to look out for:

Male fish are typically much more vivid than their female counterparts, with bolder, deeper colors, as well as having a long, thin anal fin, which you’ll find on the underside of the fish, close to the tale.

In contrast, female fish have an anal fin that is triangular or fan-shaped, found closer to the belly or abdomen, running towards its tail, and will of course be a little duller.

The Mating Dance

Just like all living creatures, fish like to seduce each other before committing to getting it on, and if you believe your fish might be pregnant - think back. Have you noticed any strange behavior lately?

Mating rituals in fish will typically see the males aggressively chasing the females around the tank, and it may look as though he is trying to attack her, resulting in bumps and scrapes.

I know you don’t want to think about your pet fish and hanky panky, but if you’ve noticed a male and a female grabbing on to each other, flipping around and upside down or wriggling against one another, you’ve likely witnessed s-e-x.

It’s pretty distinctive from their usual tank-dwelling activities, so you’ll know it when you see it, and if you remember some bizarre dance-like escapades prior to discovering the potential pregnancy, you’re probably right!

Tell-Tail Signs

One clear indicator of pregnancy is a red or black patch on the belly, called a gravid spot, a small bulge that will become larger, brighter and more obvious to the eye over the course of 20 to 40 days of pregnancy.

If you aren’t sure straight away, keep an eye on it for a while - if your female is pregnant, their bulge will only grow over time, never shrinking, so be sure too look carefully.

Another sign to watch out for is a fish struggling to swim because of weight gain, impairing its mobility; should she be hanging out on the floor of the tank, and definitely not just an over-eating male, she’s probably pregnant.

Pregnancy is traumatic whether you’re a human or an animal, so you might notice your fish hiding away behind any trinkets or plants in their tanks, possibly seeking some alone time or scouting out a safe place to deliver when ready.

Much like expectant parents, fish go through the nesting process, which is a clear sign that labor is only a few days away - if you notice she’s getting to know the decor pretty well, it’s time to prepare for labor.

Fish especially close to giving birth might begin to go off their food or refuse to eat anything altogether - though this is only definitely a sign of pregnancy in conjunction with the other symptoms, and may indicate something else without them.

Something More Sinister?

There is a serious illness in fish that causes bloating, commonly referred to as dropsy. This is an ailment that impacts fish in captivity, a serious condition that is easily mistaken for pregnancy because of their similar symptoms.

Abdomen swelling, a result of kidney failure in fish which causes their bodies to retain excess water, is the clearest indicator of dropsy, though there is not the same change in color, and the bloating isn’t localized in one key area.

Other indicators of dropsy could be…

  • bulging of the eye
  • swimming towards the surface of the water
  • curvature of the spine
  • lethargy or lack of enthusiasm for swimming. 
  • a loss of appetite (see, we told you!_)
  • red spots or sores
  • Long, white feces
  • paler, duller appearance
  • gasping for air

Cancerous Tumors?

It’s very difficult to diagnose a fish with a tumor, cancerous or otherwise, because they can appear to be so many other things. In fish, it’s very likely that a tumor will be benign, or non-problematic, and requires no treatment.

However, in the unfortunate event that your finned friend does have a malignant tumor, there’s very little chance of their survival because of a lack of treatment; always consult a fish vet before diagnosing yourself, though. 

Is My Fish Just Fat?

When you’ve noticed a considerable change in the size of your fish, but there are no other indicators of pregnancy or illness, congratulations! It’s very likely your fish is fine and has just been eating too much.

The important thing is not to increase their food intake, and adhere to their recommended diet - even though they’ve grown, this doesn’t mean they need more food than they were previously consuming.

It’s very possible to overfeed your fish, which can cause serious issues down the line - an overfed fish is going to produce feces more often, which increases the levels of ammonia in the tank and reduces the overall oxygen.

Have you seen Nemo and friends gasping for air towards the surface of the water, as though they can’t breathe? This indicates an issue in the tank, potentially as a result of feeding too much or providing a low-quality diet.