How to Tell if a Guppy Is Pregnant (and When She'll Give Birth)

First check: is it a male or a female?

Only a female can be pregnant, so a fish with a male’s anal fin isn’t pregnant, whatever its belly looks like.

Look at the anal fin, the one behind the belly. A male’s is a rod-like structure called the gonopodium. A female’s is fan-shaped or triangular 2. Males are also smaller and more colorful, while females are larger and nearly colorless apart from the tail 2.

Signs a guppy is pregnant

Start with the gravid spot, a dark patch toward the back of the belly. Females usually have one anyway, but it gets larger and more noticeable when she’s pregnant 2. In dark or black varieties it can be nearly impossible to see 2.

The spot alone doesn’t settle it. Keepers point out that a female that has been with a male may carry stored sperm without being pregnant right now, so the belly matters too 8.

Keepers say the sure sign is the belly filling out and squaring off near the rear 7. Close to birth, you can sometimes see the babies’ eyes through the gravid spot 2.

A shy female who stays near the heater is another hobbyist clue, though it’s anecdotal 8.

How long does guppy pregnancy last?

Plan on 4 to 6 weeks 1.

The numbers vary a bit. Aquarium Co-op says females give birth about every 30 days 3, and a St Andrews lab study averaged 35 to 44 days 5.

Temperature changes the pace, too. Warmer water speeds breeding up and cooler water slows it down 3.

Can one mating make several broods?

Yes. A female stores sperm in her oviduct and can keep having young for 6 months or more without a male 1. Researchers describe her producing a series of litters from a single insemination 5.

So moving a pregnant female away from the male doesn’t end it. Keepers report she can still drop more than one batch of fry 8.

How many babies will she have?

Anywhere from 5 to 100 per brood, according to Seriously Fish 1. In the St Andrews study the averages were smaller, about 5 to 9 per brood 5. Have cover ready for a big brood.

Before the birth: protect the fry

Guppies eat their young, often right away, and so do other fish 2. Seriously Fish suggests keeping adults and fry apart after the birth 1.

Dense cover is the easy fix. Live plants such as hornwort give newborns places to hide, and Java moss works too 2. Use a gentle sponge filter or a sponge over the intake so the fry don’t get sucked in 3.

Keep 2 to 3 females per male. That spreads out the mating harassment, which stresses females 2. Our stocking guide covers how many guppies fit a 10 or 20 gallon.

Breeder box: worth it?

A breeder box is a net or box that hangs in the tank and keeps fry apart from adults. The fry share the tank’s water, and for livebearers they can stay about 4 to 5 weeks, until they’re too big to fit in an adult’s mouth 4. Many hobbyists put the pregnant female in one to give birth, then take her out 2.

The downside is stress. Forum keepers report that moving a pregnant female into a box or a separate tank can make her reabsorb or hold the fry 7. We’d try plants first. If you want a box, put the newborn fry in it after the birth.

After the birth

Newborn guppies are born fully developed and free-swimming, and they’ll eat just about any food you offer 2.

Feed them often and in tiny amounts. Aquarium Co-op gives fry three to five small meals a day, each much smaller than an adult’s, so the water doesn’t foul 3. Keep up with partial changes, as covered in our water change guide.

Move them back with the adults once they’re too big to fit in an adult’s mouth, about 4 to 5 weeks for livebearer fry 4.

For everyday care once the fry are grown, our guppy care guide covers water, temperature, tank mates and why guppies from a store so often die.

Pregnant or just swollen?

A round belly isn’t always fry. Check the rest of the fish.

• Pregnant: a growing gravid spot and a belly that squares off 2. The fish otherwise looks healthy, with no sores or ragged fins.
• Overfed or constipated: Aquarium Co-op warns that guppies beg and are easy to overfeed, which can cause constipation. Feed adults once or twice a day, what they finish in about a minute 3.
• Dropsy: the Merck Veterinary Manual describes fluid building up in the abdomen with bacterial infection, along with bloody spots or ulcers, ragged fins or enlarged eyes. The pinecone look is the giveaway 6. That’s an illness, not a pregnancy. Isolate the fish and consult a fish or exotic-pet vet if one is available.

What hobbyists report

These two threads come from the Aquarium Co-op forum, where newcomers ask this exact question. Paraphrased. Neither poster had a confirmed answer when they asked, so read the replies as keepers’ rules of thumb.

• A lone female with a flickering gravid spot 7. The spot kept switching between black and red and black, but weeks passed with no fry. Replies said the spot isn’t proof, and that the sure sign is the belly squaring off. Several suspected she was stressed from living alone, since females can reabsorb or hold fry. One member warned that a mesh isolation box would likely make it worse and told them to give her company, plants and dim lights. After a male joined her, she dropped about 26 fry.
• A new keeper’s female in a separate tank 8. Replies said the dark spot alone only shows she’s ready to breed and may carry sperm. Look for a very dark spot, a rounded belly, and, as she gets close, a shy fish that hovers near the heater. Another member said the spot lightens and shows tiny dark eyes, and that she’s close once the belly squares off. The next day the poster reported she’d been ready, and others took it as a good sign she’d been pregnant.

FAQ

Can a guppy get pregnant without a male? Not on its own, but she can have broods for 6 months or more from stored sperm 1.

How long are guppies pregnant? About 4 to 6 weeks 1.

How many babies do guppies have at once? Anywhere from 5 to 100 1, though a lab study averaged far fewer 5.

How often do guppies give birth? About every 30 days under the right conditions 3.

Do guppies eat their babies? Yes. Provide dense plants or move the fry out 2.

What does a pregnant guppy look like? A larger, darker gravid spot, a fuller belly that squares off, and sometimes visible fry eyes near birth 2.

Should I separate a pregnant guppy? We’d add plants first. A box can protect fry, but forum keepers report that moving the mother can stress her 7.