The sex (or gender) of an infant is determined by the pair of sex chromosomes that he or she inherits. While eggs or oocytes carry a single X chromosome, sperm carry either an X or Y chromosome. The combination of XX inherited from both parents results in a female infant. The combination of XY results in a male infant. In nature, there is a 1:1 ratio of X and Y bearing sperm, resulting in approximately equal numbers of male and female offspring. Preconception sex selection refers to any procedure attempting to influence the sex of offspring before pregnancy.
The process of sex selection was initially devised to select for female infants when one parent carries an X-chromosome linked genetic disorder. In most of such cases, female children are unaffected carriers while male children are affected by disease. More recently, sex selection has also been used for family balancing; in families with one or more children of a particular sex, sex selection can increase the odds of having a child of the other sex. Techniques include several methods of sperm processing to enrich for a particular sex chromosome. These include using flow cytometry or various density gradients to separate heavier X sperm containing more genetic material from lighter Y sperm containing less genetic material. Success rates for the most common sperm separation techniques generally range from 60-75% when enriching for male sex, and 70-85% when enriching for female.
Over the past decade, a new investigational technique called preimplantation genetic diagnosis (PGD) has emerged as the most effective method of sex selection. In the PGD process, the patient undergoes in vitro fertilization (IVF) to enable testing of several embryos. A single cell or blastomere removed from each embryo is then tested to detect X and Y chromosome material. Only embryos of the desired sex are then transferred , achieving up to 100% correct sex identification and selection.
Families interested in preconception sex selection for various indications are encouraged to speak with their fertility doctors to discuss the success rates, effectiveness, risks and benefits of the many options available.