Old wives' tale redux

"Horrible as it sounds, in case anything had happened to him, I would have liked to let him know he was having a son or a daughter," said Rivera.

The test was right. They had a boy.

Adinda DeBoevere, a mother of three boys in Novato, Calif., wanted a girl so badly that she and her husband spent $25,000 on in-vitro fertilization so that doctors could select female embryos to implant in her womb.

"You keep on asking, 'Did it work? Did they put the right embryo in?' " she said. To find out, the former criminologist took a DNA test when she was 10 weeks along.

The test said a boy. They had a girl.

Scientific studies

A baby's gender is determined by one of the 23 pairs of chromosomes in the human genome. Mothers always contribute an X chromosome. If the father provides another X, the baby is a girl. If the father supplies a Y, the baby is a boy.

Scientific studies have found that a pregnant woman's blood contains a small amount of fetal DNA, and the gender tests claim they can detect signs of the Y chromosome even if the embryo is no bigger than a grain of rice.

The problem is that divining traces of DNA from maternal blood is not simple.

Science has tried for more than a decade to find a simple and accurate way to determine gender early -- and largely failed, using the most advanced technology available.

In a 2004 study, five medical centers in a National Institutes of Health consortium received identical blood samples from 100 women who were 10 to 20 weeks pregnant.

The centers used the same method to look for Y chromosomes in the maternal blood, but none was able to detect all of the 35 fetuses known to be male. According to the study, the detection rates ranged from 31% to 97%.

Italian researchers published a study in 2005 demonstrating that they could correctly identify 98.7% of boys and 100% of girls by looking for male DNA in maternal blood drawn during the first trimester. But their method required a relatively large blood sample -- 10 milliliters -- that was processed within a few hours.

The companies, in contrast, require just three to 10 drops of dried blood, which can take days to arrive through the mail.

Laura Cremonesi, senior author of the Italian study, said that she doesn't know anything about the companies' laboratory procedures and has no idea if their methods would work.