Fish Gestation Period: How Long Are Fish Pregnant?
Livebearing fish genuinely do gestate, so the term is correct for mollies, guppies, platies and swordtails: the eggs are fertilised and develop inside the female, and she gives birth to free-swimming fry after about 28 days. Egg-laying fish are different. A betta, goldfish, angelfish or danio releases eggs that are fertilised and incubate outside the body, hatching in two to seven days.
Molly fish pregnancy calculator
Mollies are livebearers, so this calculator works like a true gestation calculator: enter the date your molly mated and it returns the expected birth date, using the 28-day average with a 20 to 40 day range. The full comparison charts for every species follow below.
Molly pregnancy calculator
Expected birth date
Saturday, 10 October 2026
Earliest
2 Oct 2026
Latest
22 Oct 2026
Do fish have a gestation period?
Some do. Livebearers fertilise eggs internally and carry them until the young are ready to swim, so gestation is the right word and it lasts roughly four weeks. Most aquarium fish are egg layers: the eggs leave the body before development starts, so what you are timing is incubation, measured in days rather than weeks. Because fish are cold blooded, water temperature moves both figures more than any other factor.
Molly fish gestation period
A molly is pregnant for about 28 days, with a normal range of 20 to 40 days. Warm water near 28°C shortens it to roughly three weeks; water near 22°C stretches it towards 40 days. A single female commonly delivers 20 to 60 fry and can store sperm for months, so further broods arrive without another male present.
Guppy, platy and swordtail gestation
| Species | Average days | Range | Brood |
|---|---|---|---|
| Molly | 28 | 20–40 | 20-60 fry |
| Guppy | 28 | 21–30 | 20-50 fry |
| Platy | 28 | 24–30 | 20-40 fry |
| Swordtail | 28 | 24–30 | 20-80 fry |
Egg-laying fish: incubation instead of gestation
| Species | Days to hatch | Range | Eggs |
|---|---|---|---|
| Betta | 3 | 2–3 | 30-500 eggs |
| Goldfish | 5 | 4–7 | hundreds of eggs |
| Angelfish | 3 | 2–3 | 100-500 eggs |
| Zebra danio | 3 | 2–4 | 100-200 eggs |
Signs a fish is about to give birth
- The abdomen squares off rather than staying rounded.
- The gravid spot near the vent darkens and enlarges.
- She hides among plants or hangs near the heater and stops eating.
- Breathing quickens and she may shimmy or stay still in one corner.
- Fry are often delivered over several hours, usually early in the morning.
- Molly: Mollies carry developing fry internally and give birth to free-swimming young. A gravid spot near the vent darkens as the fry develop.
- Guppy: Females store sperm and can produce several broods from a single mating.
- Platy: Platies are among the easiest livebearers to breed and rarely need intervention.
- Swordtail: Larger females carry bigger broods and benefit from dense planting to shelter the fry.
- Betta: The male builds a bubble nest and tends the eggs until they hatch and the fry are free-swimming.
- Goldfish: Eggs are scattered over plants and spawning mops, and hatch faster in warmer water.
- Angelfish: Pairs lay on a vertical surface such as a leaf or slate and guard the eggs together.
- Zebra danio: Danios scatter eggs and will eat them, so a spawning grid or marbles protects the clutch.