The best thing to do when you have pregnant livebearers is to keep them calm and relaxed and have lots of plants in the tank. The females hide in the plants when they are giving birth and the babies can hide in them too. Live plants are best but plastic will do.
Some good live plants to try include Ambulia, Hygrophila polysperma & H. ruba/ rubra, Elodia/ Hydrilla, narrow Vallis, common Amazon Sword plant, and Water Sprite (Ceratopteris thalictroides/ cornuta).
The water sprite is a floating plant but can also be planted in the substrate. The other plants should be planted in the gravel.
Out of all the plants available, Water Sprite is the best plant for livebearers.
Have the light on for 8-12 hours a day and let the plants grow. The females and babies will use them.
-----------------------
Do not put pregnant female livebearers in breeding nets because they stress out due to the small space and can give birth to premature babies that die.
Once the females have had the babies, you can use a small plastic container to scoop them out (with some water) and put the babies in the breeding net.