The short answer
Only livebearers — guppies, mollies, platies and swordtails — actually get “pregnant” and give live birth. The two classic signs are a rounded, swollen belly and a dark gravid spot near the vent that grows and darkens as the fry develop. Egg-laying fish don’t get pregnant; they just fill with eggs (become “gravid”) before spawning and releasing them.
Reading a livebearer
In a female guppy or platy, watch for:
- A boxy, swollen abdomen that becomes squarer as the fry grow.
- A darkening gravid spot — the patch near the rear vent. In late pregnancy you can sometimes see tiny eyes in it.
- Behaviour changes near her due date — hiding, hovering near warmth or the substrate, and less appetite.
Roughly 28 days after mating she’ll deliver a batch of free-swimming fry. For the species detail, see our guppy care guide and how long are guppies pregnant.
Gravid egg-layers
Fish like corydoras and angelfish swell with eggs before spawning rather than carrying young. A gravid female corydoras looks plump from above; angelfish develop a visible breeding tube shortly before laying. They then deposit eggs on a surface to be fertilised — see how do I breed corydoras and how do I breed angelfish.
Getting ready for fry
If a livebearer is close to term, add dense cover or move her to a breeding box so the fry aren’t eaten at birth — the parents will eat them otherwise. Have a plan and some powdered fry food ready. See what do I do with baby fish, what do I feed baby fish, our food picks, and browse aquariums for a grow-out tank.