Blood tests that isolate fetal DNA from a pregnant woman's blood can detect gender after seven weeks of gestation and are better than ultrasound at ruling out some genetic abnormalities, according to a new report.
Ultrasound and urine tests are unreliable at determining fetal gender at that stage of development, according to a review of studies published in today's Journal of the American Medical Association.
Amniocentesis, another test that involves using a needle to remove and check amniotic fluid, carries a risk of miscarriage and can't be used until at least 15 weeks.
The blood test can help women who are carriers of some hereditary diseases, such as hemophilia, avoid unnecessary invasive tests if the fetus is female. Males are more susceptible to diseases on the X chromosome because they only have one copy. Females have two, inherited from both parents, and can rely on a healthy one if the other is damaged.
"If you know in the first trimester that there's no male DNA, that the fetus is female, then an invasive procedure isn't needed," said Diana Bianchi, a study author and the executive director for the Mother Infant Research Institute at Tufts Medical Center in Boston.
The blood tests are made by companies including San Diego-based Sequenom and Consumer Genetics of Santa Clara, Calif.
The researchers, led by Stephanie Devaney at the National Institutes of Health in Bethesda, Md., analyzed 57 previous studies representing 3,524 male-bearing pregnancies and 3,017 female ones. They found that the tests, which detect the male Y chromosome in the maternal bloodstream, have a sensitivity of 95.4 percent.
Others who may benefit from screening are those at risk for congenital adrenal hyperplasia. Girls with the condition have ambiguous genitalia, according to the National Institutes of Health. That can be prevented in the womb by injecting the mother with steroids. Boys don't need the treatment, Bianchi said. Knowing the sex early can cut down on the number of women treated with steroids they don't need, she said.
The blood tests probably won't change sex-selective abortion in the United States, according to Bianchi.
"It's a theoretical issue now, but people are already doing it using ultrasound," she said.