A gravid spot is the dark patch that appears near the rear of the belly on a pregnant female livebearer — such as a guppy, molly or platy. It darkens and enlarges as her pregnancy progresses, and is the clearest visible sign that she is carrying developing fry.

Here is how to identify it and what it tells you.

What the gravid spot shows

The gravid spot is the female’s ovarian region showing through the body wall. As eggs are fertilised and fry develop internally, the area darkens from pale to deep grey or black. In the final days you can sometimes see individual eyespots of the developing fry through it.

Livebearers give birth to free-swimming young rather than laying eggs — see breeding livebearers.

How to identify it by species

• Guppies: clearest example — a dark spot behind the anal fin that intensifies over 3–4 weeks.
• Platies: visible but subtler; the female also squares off when viewed from above.
• Mollies: hardest to see because of their deeper bodies; behaviour is a better cue.

What to do when you see one

A darkening gravid spot plus a boxy, swollen belly means birth is near — usually within days. Provide dense plants or a mesh breeder net so fry can hide, and read how to breed guppies for fry care.

Frequently asked questions

What is a gravid spot on a fish?

It is the dark patch near the rear of the belly on a pregnant female livebearer such as a guppy, molly or platy. It shows the ovarian region and darkens as fry develop.

How can you tell if a guppy is pregnant?

Look for a darkening gravid spot behind the anal fin and a swelling, boxy belly. Near term, the spot deepens and the female may hide and hover in one spot.

How long after the gravid spot darkens does a fish give birth?

When the gravid spot is very dark and the belly is boxy and swollen, birth is usually within a few days. Guppy gestation lasts about 21–30 days in total.

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— Jaqline Smalling, FishForever