It's a boy! It's a girl! Until the 1970s, these words welcomed virtually every child into the world. In less than one generation, however, new reproductive technologies have shifted this announcement from the delivery room to the obstetricians' office; ultrasounds and amniocenteses now allow expecting parents to choose their nursery walls' paint color months before giving birth.

The science and business of sex identification took yet another quantum leap forward this past week with the Pregnancystore.com's release of the Baby Gender Mentor Home DNA Gender Testing Kit. Now, a woman can know her child's sex shortly after she discovers her pregnancy. As soon as five weeks after conception, she can prick her finger, FedEx a blood sample to Acu-Gen Biolab in Lowell, MA, and have the sex of her sprouting embryo emailed to her faster than Netflix can send her next movie.

An unequivocal good, many say. What's more harmless than learning whether to buy dolls or trucks for the toy chest?

Ultrasounds and amniocenteses cannot accurately determine a fetus' sex until at least four months into pregnancy and sometimes...