It's safe to assume that all female livebearers are pregnant, all the time; they can also store 'packets' of sperm, so they can continue getting pregnant, even if there are no males in the tank.
Your little female is a long way from giving birth, as she isn't very big yet, although, as she's young, she might not get as big, or have as many fry, as a mature female.
When she's near to giving birth, she'll develop a very square shape to her belly.
We don't recommend the use of birthing nets/breeder boxes, as they are too small for the female to feel comfortable, and too small for the fry to grow properly. Leaving the fish in the main tank is best, unless you have a spare tank of around ten gallons to use as a nursery.
Add lots of live plants for hiding places if you want some fry to survive.