Couples who try for one sex or another when conceiving a baby experience mixed results with traditional methods, but if you don’t want in vitro fertilization, there is one persistent theory. The man’s sperm is what determines the sex of your child, and sperm that make boys are said to be much faster, but also more vulnerable to an acidic environment than the ones that make girls. This is why some women use a vinegar-and-water douche prior to sex, and others suggest not having sex after about the third day of your fertility window, allowing the boy sperm to die off and girls to have a chance at fertilizing the egg.
X linked chromosomes (the one which lets you have a girl) have been said to love acidity which can be found closest to the openng of the vagina. Y linked chromosomes have been said to become weakened by this acidity. Therefore if the theory is correct, it would help your cause of you deposit the sperm close to the base of the vagina.