The quest to identify fetuses carrying potentially fatal genetic diseases got a powerful tool in the early 1990s, with the development of Pre-Implantation Genetic Diagnosis. PGD, as it's known, allows doctors to scan the cells of an embryo fertilized outside the womb for signs of inherited diseases like cystic fibrosis or sickle-cell anemia, giving parents the option of not implanting that embryo.

In some fertility clinics, PGD is increasingly being used only to determine the embryos' sex, and then just boys, or girls, are selected to develop into babies. Dr. Daniel Potter offers his patients this controversial service at the Huntington Reproductive Center in Southern California. In his practice, he finds that immigrant couples are making different choices for children than American-born couples.

The procedure takes place in the Center's operating room. The patient is anesthetized, and the reproductive specialist extracts her eggs. They will be fertilized outside her body, even if the woman is completely capable of getting pregnant naturally.

Dr. Potter will analyze the chromosomes of her embryos, and select only the sex she's requested. "Virtually 100% of...