Skip to Content

How Long Are Molly Fish Pregnant?

Disclosure: When you purchase something through my affiliate links, I earn a small commission. As an Amazon Associate, I earn from qualifying purchases.

As I knew my molly fish was pregnant, I immediately wondered how long will this pregnancy take. I heard that it depends on many factors, but I still wanted a general estimation. Luckily, over the years, I gained some experience in that field.

Molly fish are typically pregnant for roughly 50 to 70 days. Yet, that period may shorten if the molly is provided with hiding places and ideal water conditions. Also, the gestation period of some molly species is shorter, such as sailfin mollies, which are pregnant for merely 21 days.

As we move forward, I will go into the different factors that impact the gestation period of molly fish. Then, I will share a few tips to ensure that your pregnant molly delivers in time. Following these tips will also increase the survival rates of the fry.

Still curious? Feel free to check my complete guide on pregnant molly fish. There, I discussed how to care for pregnant mollies, how long they remain pregnant, how to identify signs of pregnancy, and a lot more.

How Long Are Molly Fish Pregnant?

Female mollies are livebearers. A male fish has to inseminate them before they can produce live fry. This matters because it can affect your expectations regarding a molly’s gestation period.

Molly fish are pregnant for roughly 50 to 70 days.[1] For some fish, that figure is closer to 30 days. For others, it may exceed 70 days. Don’t panic if your molly fish have been pregnant for longer than expected. 

Take these factors into account:

1. It Takes Time For Mollies To Mate

The gestation period starts when the male fish fertilizes the eggs in the female fish. It ends when the female molly fish gives birth. While this sounds like common sense, many people forget to ignore the mating stage. 

They expect those 50-70 days to include the entire breeding process, including that initial mating phase. But you can’t include the courtship stage in the gestation period because the molly fish is not yet pregnant.

You can tell that a pair of mollies is courting because the male molly will chase the female molly. Depending on the type, the male fish may even change color. Courtship may continue for several days before the female fish conceives.[2] 

You can’t start counting until insemination occurs. Some people confuse mating behavior between courting mollies with aggression. Those mannerisms will most likely subside once the female molly falls pregnant.

2. You Bought A Pregnant Molly Fish

You have to remember that female mollies can give birth without a male fish. They still require a male fish to inseminate them. But once they mate, the females can store the sperm, using it to fertilize their eggs down the line.

Female mollies can use that sperm to produce up to five batches of baby mollies without the direct involvement of a male fish.[3] This can easily confuse beginners who may wonder why their female mollies are giving birth so soon after mating with a male molly fish.

They don’t realize that those female mollies were probably pregnant before they mated with the male molly. Some females may give birth mere days after arriving in a home tank, which can trick an amateur into believing that their molly was pregnant for just a few days.

In many cases, an experienced aquarist has to inform these newcomers that their mollies were probably pregnant before they entered the home tank. 

The best way to determine the gestation period of your mollies is to calculate the average gestation periods of multiple female mollies.

Otherwise, you have no way of knowing whether individual mollies were inseminated in the tank under your watch or if they conceived weeks before you brought them home.

3. There Are Different Types Of Molly Fish

You can find molly fish in various types. The gestation period will vary depending on the kind of molly fish. For instance, black lyretail mollies have a long gestation period that can last for 60 days.

On the other hand, white sailfin mollies are pregnant for just 21 days. That figure jumps to 30 days for common black mollies and 40 days for black sailfin mollies.[4] However, don’t expect the mollies to adhere strictly to these numbers. 

A black lyretail molly could give birth within 30 days if the conditions are appropriate. White sailfin mollies may remain pregnant for over 60 days. This is why many professionals emphasize the 50 to 70-day estimate.

The gestation period of a livebearer falls within a relatively wide range. These creatures are susceptible to environmental factors that can shorten or lengthen the gestation period.

For instance, an optimal temperature of 77-80 degrees F can encourage these fish to deliver their babies within four weeks. But at 68 degrees F, they may remain pregnant for 35 or more days. 

The same thing can happen if the lighting is dull and unreliable.[5] In fact, poor lighting can prevent the fish from reproducing altogether because the absence of light is reminiscent of winter conditions. 

4. The Aquarium Conditions Matter

The wrong conditions can discourage mating in a molly fish tank. As was inferred above, the wrong conditions can also cause delays among pregnant fish. A molly fish may extend its pregnancy, refusing to give birth because of the stress exerted by its environment.

You see this in tanks with the wrong parameters, insufficient hiding places, and aggressive tankmates. Female mollies will go into hiding as they approach their due date because they want to give birth in a secure environment.

But if they don’t feel secure, they will keep their fry inside the womb for a few more hours, days, or weeks. This doesn’t come as a surprise to most aquarists. However, many do not realize that pregnant mollies are not obligated to give birth.

They can refuse to give birth. You see this in aquariums with perpetually terrible conditions. If a molly fish concludes that the conditions in the tank won’t improve, it may abort the pregnancy altogether.

This is where it reabsorbs the fry. Contrary to popular belief, the re-absorption process doesn’t turn the fry back into eggs. Instead, the female fish reabsorbs the baby mollies as nutrients. It can make more eggs down the line.

5. Molly Fish Can’t Be Pregnant Forever

At the moment, no one has identified the longest period for which a molly fish can remain pregnant. But if your pregnant mollies have exceeded the 70-day threshold, you have to assume that they aborted or miscarried. 

In this case, the gestation period is over, and you can stop counting. However, this isn’t true in every single case. Some pregnant mollies may give birth without your knowledge. The fry may go unnoticed if you have a large community tank with numerous fish.

Also, if you wait too long, the adult fish may eat all the fry before you can take note of their presence. This is the primary reason why aquarists keep adult mollies and their younger counterparts separately. Once it gives birth to them, a female molly will happily eat its young ones

Neither scenario has warning signs. You have no way of knowing whether a female molly aborted its young ones or if it gave birth to fry and then ate them all before you noticed. The molly’s swollen stomach will shrink back to its original size in both cases.

How Do I Ensure That My Molly Gives Birth In Time?

As was noted earlier, environmental factors have a significant impact on the gestation period of your molly fish. As a rule of thumb, your ultimate goal is to relieve stress as much as possible. Also, the water chemistry should be ideal.

Try to maintain these water parameters:

  • Temperature: 77-80 degrees F (25-27 degrees C)
  • pH: 6.7 and 8.5
  • Hardness: 20-30 KH
  • Ammonia & Nitrites: 0 ppm
  • Nitrates: <20 ppm

To measure the pH, ammonia, nitrates, and nitrites, I personally use the API Water Test Kit (link to Amazon). I found this one to be the most accurate. It also lasts for about eight hundred measures, making it highly cost-effective.

I also suggest that you avoid aggressive tankmates. Avoid species like bettas, goldfish, barbs, and cichlids.[6] These will probably stress your pregnant molly and delay her delivery. 

I also recommend that you introduce plenty of hiding spaces. Pregnant molly fish take advantage of plants, caves, and decorations when they are about to spawn. These will also serve the newborns, as they can quickly be eaten by other fish, including their mother.

If you found this article helpful, these may also interest you:

Conclusions

Molly fish are typically pregnant for 50 to 70 days. But you should also consider the mating phase, which could last for about a week. During this time, the male molly fish will chase the female counterpart. Once the female is conceived, she might be chasing the male.

To ensure that your molly delivers in time, try to maintain the proper water conditions. The most crucial factor would be the temperature. I also suggest that you check the pH, nitrates, nitrites, and ammonia.

Inappropriate water conditions will stress your molly fish, which may eventually reabsorb the fry. If you chose a different tank for spawning, use the same water the original tank contains. Avoid drastic changes with pregnant mollies.

References

  1. https://www.swelluk.com/blog/how-long-are-tropical-fish-pregnant-for/
  2. https://aquariawise.com/how-long-molly-fish-pregnant-for/
  3. https://lifeoffish.com/how-long-are-mollies-pregnant-for-12-things-to-know/
  4. https://www.aquariumfocus.com/how-long-does-it-take-for-a-molly-to-give-birth/
  5. https://www.thesprucepets.com/live-bearer-development-period-4040417
  6. https://www.aquariumnexus.com/best-molly-fish-tank-mates/