Not only television and videos reenforce stereotypes of active, competitive boys and passive, nurturant girls. Parents, peers, and teachers also act differently toward boys and girls, and children, from an early age, learn that it isn't cool to play with children of the other sex. A child's biological sex is determined by whether the sperm cell with which an egg fuses contains an X or a Y chromosome. If the sperm is X, the child will be a girl, if Y, it will be a boy. Nowadays there are techniques for enriching sperm samples so that they contain more cells of one kind than the other.
None is perfect because the two kinds of sperm are very similar.
All one can do is slant the probabilities. There are specific health reasons for wanting to do that. If a woman has a family history of a so-called sex linked condition, such as Duchenne muscular dystrophy or fragile X syndrome, it is more likely to show up in boys than in girls. Therefore, one way to avoid passing on such conditions is to have only girls. Once a woman is pregnant, that means terminating the pregnancy if the fetus is male.