It has been known for many years that the gender of a child is determined by the sex chromosome carried by the sperm. Sperm bearing an "X" chromosome (men have both "X" and "Y" bearing sperm), when united with the "X" chromosome from the female’s egg (females only produce "X") will result in an "XX" pregnancy that produces a female. If a sperm bearing a "Y" chromosome unites with the "X" chromosome from the female, an "XY" pregnancy will give rise to a male. Therefore, with a natural pregnancy or with assisted reproductive technologies, such as IUI or IVF, each pregnancy has a 50% (or 1 in 2) chance to be male and a 50% chance to be female. Gender selection/sex selection may be used when parents need/want a child of a specific sex. Gender selection can help prevent the transmission of certain genetic diseases. Genetic diseases carried on the X or Y chromsome are termed “sex linked” genetic diseases. Most commonly, gender selection is performed in instances where serious sex-linked diseases, such as hemophilia or muscular dystrophy are a concern; or when some couples have children of one gender, and choose elective “ family balancing” to help them have a child of the other gender.