Blood tests that isolate fetal DNA from a mother’s blood to detect gender work after seven weeks of gestation and are better than ultrasound at ruling out some genetic abnormalities, according to a report.
Ultrasound and urine tests are unreliable at determining fetal gender at that stage of development, according to a review of studies in the Journal of the American Medical Association. Amniocentesis, which involves using a needle to remove and test amniotic fluid, carries a risk of miscarriage and can’t be used until at least 15 weeks, after the end of the first trimester.
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.
The tests are made by companies including San Diego-based Sequenom Inc., and Consumer Genetics Inc. of Santa Clara, California.
The researchers, led by Stephanie Devaney at the National Institutes of Health in Bethesda, Maryland, analyzed 57 previous studies representing 3,524 male-bearing pregnancies and 3,017 female ones. They found that the tests, which detect the 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 newer tests probably won’t change sex-selective abortion in the U.S., according to Bianchi.
“It’s a theoretical issue now, but people are already doing it using ultrasound,” Bianchi said.