If you notice a dark, square-shaped spot near the anal fin of your female mollies, you’ve spotted the gravid spot. This black markings indicates that she is pregnant with a fresh brood of fry and will give birth soon.

The gravid spot on mollies forms during pregnancy when the female’s body starts making eggs. As the eggs develop inside her, they cast a shadow on the skin near her back end. This shadow shows through as a black, grainy patch – the gravid spot.

Read on to learn all about this special pregnancy marking, what it signals about your molly’s gestation period, and how to care for an expecting molly fish.

What Exactly is the Molly Fish’s Gravid Spot?

The gravid spot on a female molly fish is a dark marking that appears near her anus as her eggs develop internally. This unique marking signals that she is pregnant and about to give birth to a brood of live fry.

Understanding this interesting feature can help molly fish owners better care for their pregnant fish.

A Dark Marking Near the Anus

The gravid spot is a dark, oval-shaped area that forms on the rear underside of a pregnant molly, just in front of her anal fin. It’s located right where her ovipositor would be if she was an egg-laying species.

The coloration starts out rather faint but darkens dramatically as the birth approaches, usually to an intense black or dark gray.

This marking is caused by the developing eggs inside the female’s ovary. Mollies are livebearers, meaning their fertilized eggs hatch internally and they give birth to free-swimming fry rather than laying eggs.

The gravid spot forms as the eggs grow within the ovary, concentrated near the rear of the fish.

Forms as Eggs Develop Internally

The gravid spot starts becoming visible within a week or two after a female molly has mated. At first, it will appear as a faint gray smudge on her underside. As the eggs mature over the next 20-30 days, the marking steadily darkens and expands in size.

Right before giving birth, the spot will be very dark, almost black, and quite large/prominent. This lets an observant aquarist know that the molly will soon be delivering her fry. The dark coloration is caused by the eggs being tightly clustered together as they finish developing.

Signals Impending Birth of Fry

The prime function of the gravid spot is to signal when a pregnant molly is about to give birth. Once the female’s gravid spot is very dark, she should deliver her fry within the next few days. It’s a clear indicator that the babies are fully developed and ready to be born.

Knowing the birth is imminent allows the aquarist to make any necessary preparations. For example, the pregnant fish can be transferred to a breeding tank, delicate tankmates can be removed, and infusoria can be cultured to feed the tiny newborn fish.

After the molly gives birth, her gravid spot will rapidly fade away, returning to her normal silvery coloration. Then the cycle can start again if she breeds and becomes pregnant with another brood. The gravid spot is an incredibly useful tool for gauging the gestation period of these prolific livebearers.

When Does a Molly’s Gravid Spot Appear?

After Mating and Fertilization

A molly’s gravid spot typically becomes visible within a few days after successful mating and fertilization has occurred. The gravid spot appears as a dark shadow under the skin near the anal vent region of a pregnant female molly fish.

This skin discoloration is caused by the developing eggs inside the molly’s ovary that are getting ready for the gestation period.

Around 3 Weeks into Gestation Period

The gestation period for mollies is around 21-28 days. So by 3 weeks into pregnancy, the gravid spot will become very prominent on a pregnant molly. At this stage, there may be 30-60 mature eggs developing internally.

As the eggs grow bigger in size and greater in number during the 3 week period, it causes a dark mass to become visible through the translucent skin near the molly’s anus.

Egg Development Causes Skin Shadowing

The technical term for a gravid spot is a “skin shadow”. It is the skin’s shadow cast by the opaque eggs aggregating together inside the ovarian cavity close to the exterior of a pregnant molly’s body.

So as the fertilized eggs undergo cellular division and develop from single cells into mature shelled eggs with yolks, it leads to skin pigmentation right above the ovary due to tissue stretching.

The color and size of the gravid spot varies based on the quantity and maturity stage of eggs. The more mature the eggs near birth, the larger and darker the skin shadow will appear on a pregnant female molly fish.

Some female mollies may show a black mass spanning over half the body length when they are nearly full term and ready to give birth.

How Long Before Birth After Seeing the Spot?

Appears About a Week Pre-Birth

The infamous “gravid spot” on a female molly fish typically appears about one week prior to giving birth. This darkened area is caused by the eyes of the developing fry that are visible through the thin abdominal tissue of the mother.

At this late stage of pregnancy, the baby fish are fully formed and poised to emerge soon.

During the week between the gravid spot appearing and birth, the female molly undergoes noticeable behavior changes. She may become more aggressive around tank mates, voraciously hungry, and reclusive as she prepares a suitable spot to give birth.

Owners have reported seeing the darkened gravid area begin faint and gradually intensify during this week.

Color Darkens as Birth Nears

In the final stretch before birthing the fry, the molly’s gravid mark will become very pronounced. The eyes of the babies packed into her abdomen make this area seem almost black. She may frantically dart around the tank to scope out hidden corners, plants, and other seclusion to give birth safely.

Expect the coloration to reach its darkest peak right before she isolates herself to deliver the babies. Once she has settled on a secure birthing location out of danger, the fry will emerge soon after.

This progression often happens remarkably fast—her belly can go from slightly darkened to pitch black overnight!

Spot Fades After Giving Birth

During and immediately following the birthing of a molly’s brood, the infamous “pregnancy spot” changes yet again. As each slippery fry emerges, the mother’s extended belly gradually deflates. The eyes of the offspring inside are no longer visible through the abdominal lining.

Over the next day or two as she recovers from the laborious birth, the mother molly’s gravid marking lightens back up to her normal body color. Occasionally a faint dark blemish remains for a few days.

But owners can rest assured the infamous pregnancy indicator has done its job if they spy an emptied female still doting after a fresh batch of tiny fry in the tank!

Caring for a Pregnant Molly with a Gravid Spot

Keep Her Well-Fed with Varied Diet

A pregnant molly needs more nutrients to support her growing fry, so feed her a high quality and varied diet. Increase the amount you feed her by about 25% compared to normal. Offer her a mix of freeze-dried foods like bloodworms, daphnia, and brine shrimp as well as high protein flakes and pellets.

This will give her the extra protein, vitamins, and minerals she needs during pregnancy.

Feed her smaller meals 2-3 times a day rather than one large meal to aid digestion. Also include some vegetable matter like blanched spinach or zucchini which provides fiber. Just watch that uneaten food does not decay and foul the water.

Maintain Excellent Water Quality

Pregnant mollies are sensitive to poor water quality, so diligent water changes and testing are a must. Change at least 25% of the water 2-3 times per week. Use a gravel vacuum to remove waste and do not disrupt her too much.

Test for ammonia, nitrite, nitrate, and pH regularly and keep levels ideal (ammonia and nitrite at 0ppm, nitrates under 20ppm, pH 7.0-8.0).

Consider using live plants like java fern, anubias, and java moss as they uptake nitrates and provide infusoria for fry to eat. Also run an efficient filter and use products like Seachem Prime to detoxify ammonia and nitrites between water changes.

Have a Nursery Tank Ready

You’ll want to move the pregnant molly to a separate “nursery” tank to give birth, so she is not stressed and the fry have space. A 10-20 gallon tank is sufficient. Add plenty of hiding spots with java moss, floating plants like hornwort, a sponge filter for gentle water flow, and a heater and thermometer to keep the temperature 78-82°F.

Cycling the tank ahead of time is ideal, but you can do a fishless cycle with pure ammonia too. Or transfer some filter media from the main tank to seed the nursery tank with beneficial bacteria. Test water parameters and do partial water changes to get the nursery tank ready for your expectant molly!

When to Be Concerned About the Gravid Spot

If Molting or Skin Flashing Occurs

Molting or skin flashing near the gravid spot can indicate that the female molly fish is stressed or struggling with a fungal or bacterial infection. According to the journal Aquaculture Research, skin lesions and ulcers often precede fungal and bacterial outbreaks in fish.

If you notice your molly rubbing against objects in the tank, flashing its skin, or showing signs of excess molting around the gravid spot, the water conditions may be deteriorating.

Test the water parameters including pH, ammonia, nitrites, and nitrates to identify any imbalances contributing to skin stress. Perform partial water changes using a gravel vacuum to remove waste and do not overfeed. Reduce stress by providing plenty of hiding spots and plants.

If symptoms continue despite improved water quality, treat the whole tank with an over-the-counter antifungal or antibiotic medication.

Lack of Appetite or Lethargy

Healthy molly fish should be energetic and eager to eat. If your gravid molly starts acting lethargic, hiding excessively, or ignoring food, this signals a health problem requiring attention. Parasites, infections, or poor water quality can cause appetite loss and inactivity.

Start by testing and improving water parameters if necessary through partial water changes. Provide an isolated hospital tank if symptoms are severe. Treat with antibiotics or anti-parasitic medications as recommended by your local aquarium store.

Increase aeration and temperature slightly to perk up metabolism. Offer blanched veggies or live foods to stimulate appetite in conjunction with medicating.

Spot Size or Color Changes

As a female molly nears giving birth, the gravid spot darkens and enlarges noticeably. However, abnormal changes in color or size can also indicate issues. According to Florida Museum experts, unusual enlargement, protrusion, redness, or bleeding signal a potentially serious tumorous growth.

Isolate the fish and consult an exotic veterinarian whenever the gravid spot looks questionable. Remove any aggressive tankmates stressing the molly. Improve nutrition through high-quality foods with veggies.

While not definitively proven, some hobbyists report success shrinking growths using antibiotics or antifungal baths. Unfortunately, gravid spot tumors often reoccur and can impact lifespan.


A pronounced gravid spot on a female molly signals she will soon give birth to a brood of fry. This dark patch forms as pregnancy advances and eggs develop internally, causing skin shadowing near the anus fin.

While a gravid spot is normal, keep a close eye on an expecting molly. Make sure she stays active and eats well. Have an ideal nursery setup ready for her soon-to-arrive fry. With good care, your pregnant molly will successfully deliver her young.

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