Being able to tell when a fish is about to give birth requires patience and careful observation. Some fish lay eggs while others are called live-bearers and give birth to live young. Examples of live-bearers include swordtails, fancy guppies, and mollies, among many others. Other fish in the same tank as the live-bearer may eat the young if you do not separate them upon birth.
Observe the belly to determine if it is bloated or increasing in size from week to week. Some fish have large stomachs regardless of pregnancy. But pregnant fish will usually have a square outline to their belly just before giving birth.
Look for the gravid spot on the fish's abdomen near the rear tail. The spot should appear large and dark when her eggs are fertilized. You will know your fish is close to giving birth when the spot becomes nearly black. Some fish may show white spots instead of black.
Watch your fish swim and see if the female is prone to nesting. Pregnant fish will typically slow down and act sluggish, lethargic, and less mobile when getting close to giving birth. You will also notice pregnant fish enjoy resting along your tank's plants or accessories in the days before giving birth.
Check your fish's daily diet routine. A pregnant fish nearing birth is more likely to stop eating as frequently or stop altogether. Most fish are naturally big eaters and swarm the top of the tank at the first sign of food. Pregnant fish ready to give birth are more likely to rest or swim idly by than eat.
Watch your fish to see if she shakes in an unusual way. Some fish owners call this the "shimmies." She may swim and shudder for just a few seconds several times a day during the week leading up to birth. Fish typically won't shudder in this manner at any other time than the days before birth.