“I think over the long run this has the potential of changing attitudes toward pregnancy and to family,” said Audrey R. Chapman, a bioethicist at the University of Connecticut Health Center. “Women may be less invested in their pregnancies earlier than they are later, and the question has been raised whether women will look at their pregnancies increasingly as being conditional: ‘I will keep this pregnancy only if.’ ”

Fetal sex tests have a few medical applications, allowing couples with histories of rare sex-linked disorders to avoid costly and invasive genetic testing if they learn they are expecting the other sex. But for most couples, the tests, which are unregulated, simply answer the boy-or-girl question weeks earlier than ultrasound, and in a less invasive and safer way than amniocentesis.

“Seven weeks is a different time in pregnancy,” said Dr. James Egan, a professor of obstetrics and gynecology at the University of Connecticut Health Center who was a co-author of a study on sex selection with Dr. Chapman and others. “Women haven’t had the ultrasound where you see the fetus that looks like a baby. Many people don’t even know that a woman is pregnant. And you can have a medical termination,” using pills like RU-486, which can be used at home discreetly before 10 weeks of pregnancy.

There is evidence that some Americans want to choose their babies’ sex. At the Fertility Institutes, a set of clinics in Los Angeles, New York and Guadalajara, Mexico, 85 percent of roughly 500 couples each year seek sex selection, although three-quarters of them come from overseas, said Dr. Jeffrey Steinberg, the medical director.

“It’s jumped over the past four years,” said Dr. Steinberg, whose clinics determine sex through pre-implantation genetic diagnosis, an embryo screening that also detects genetic disorders. He said that “if a woman calls to make the appointment, the couple almost always wants a female. If a man calls, they almost always want a male.”