PGD is a technique whereby a three-day-old embryo, consisting of about six cells, is tested in a lab to see if it carries a particular genetic disease. Embryos free of that disease are implanted in the mother’s womb. Introduced in the 1990s, it has allowed thousands of parents to avoid passing on deadly disorders to their children.

But PGD is starting to be used to target less-serious disorders or certain characteristics — such as a baby’s gender — that aren’t medical conditions. The next controversial step is to select physical traits for cosmetic reasons.