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Livebearing fish can hold off on birthing until they feel conditions are right for survival of the fry. The best thing to do, is change out half the tank water every few days if you have a smaller tank, say under 30 gallons. Make the new, treated tap water a bit warmer than what's already in the tank.
A little added heat and a flush of pure water, is generally enough for the female to release her fry. At this point, you need to maintain pure water conditions and feed all the fish a little more often. Well fed adult livebearers are seldom interested in the fry.
One other thing. Tanks should have a lot of floating plants in them. Hornwort is a good one. Makes good hiding places for the fry.