The sex or gender of your baby-to-be is determined at conception, when the egg and sperm meet. Human beings have 23 pairs, or sets, of chromosomes. The 23rd set is known as the sex chromosomes. In females, the sex chromosomes are XX, while in males, they are XY. The unfertilized egg carries only X, or female, chromosomes. Any given sperm may or may not carry the Y, or male, chromosome.
If the egg is fertilized by a sperm carrying only the X chromosome, the fetus gets an X from the sperm and an X from the egg — making it XX, or female. If the sperm that fertilizes the egg carries Y, the fetus gets the Y from the sperm and an X from the egg — making it XY, or male.
Some people theorize that sperm carrying the X or female chromosome are heartier and have the ability to survive longer. The sperm with the male or Y chromosomes are faster swimmers but less hearty.
While you might think that the meeting of the sperm and the egg takes place in the uterus, you're wrong! Generally, this happens in the outermost third of the fallopian tube. These cells begin to rapidly divide as they travel for days toward the uterus where they will hopefully implant in the lining.
If this is true, then the timing of intercourse during ovulation has everything to do with whether your future child is a boy or girl. If you have sex just prior to ovulation, you are more likely to have a girl, according to these theories. If you wait until as close to ovulation as possible, you increase your chances of having a boy.
While this is only theoretical, there are many families who swear by this method. If you want to try to influence the gender of your baby, it might be worth a try.