Betta fish don't become pregnant the way mammals do. When betta fish reproduce, the male and female enter into a courtship ritual. The female ejects eggs, and the male fertilizes them externally with sperm. The male then builds a bubble nest in the aquarium and places the eggs in them.
The male betta fish doesn't just fertilize the eggs: he guards them in the bubble nest until they hatch. During this time, the male will be so vigilant that he might aggress against the mother of his children. It's therefore a good idea for the pet owner to remove the female to another tank.