courtesy of Bremer Kid - This and That
Pregnancy in live bearing fish is much different and less traumatic than the same process in mammals. Fertilized eggs develop inside the female fish without a placental connection, and they have no parenting instinct. In fact, live bearing parents will eat their unprotected fry.
Most livebearing female fish drop fry about once in every 28 days, and they can deliver up to three batches after only one contact with a male. As a result, mature females remain pregnant so long as they are healthy. Right before dropping fry, the gravid area near her vent becomes flattened.
In normal circumstances, live bearing fry enter the world, often at night, as fully formed individuals capable of living on their own. Their first instinct is to swim to the water surface. A large raft of fast growing plants at the water surface will provide the fry a refuge as well as some microorganisms to eat. Plants like hornwort, water sprite and anacharis also improve water quality by ingesting toxic nitrogen compounds.
A female live bearing fish placed in a breeding trap or moved to another aquarium is likely to suffer stress. This in turn can lead to premature delivery, and fry are born with yolk sacs not yet fully consumed. Often they fall to the bottom and drown. Dense floating plants eliminate the need to inflict stress on the female.