August 9th, 2011
04:00 PM ET
A blood test designed to tell the sex of an unborn baby is very reliable, especially after seven weeks' gestation, compared with urine-based tests that are also available to parents, according to new research in this week's issue of Journal of the American Medical Association.
The test based on cell-free fetal DNA taken from the mother's blood, was found to correctly detect a male fetus 95% of the time, when performed seven to 20 weeks’ gestation and then had a near-perfect rate of determining the sex of the child after that.
The blood test was designed to be an alternative to invasive procedures, such as amniocentesis, which involves removing a sample of amniotic fluid surrounding the fetus, and chorionic biopsies, which test tissue that makes up most of the placenta. These tests detect inherited birth problems like hemophilia, as well as single -gene disorders such as congenital adrenal hyperplasia and ambiguous genitalia (where a child could have a mix of both sex organs).
Authors of the paper looked at 146 studies and pieces of literature published on the effectiveness of the noninvasive prenatal test, and found it to be very accurate, especially as the gestational age of the child became greater.
One of the main concerns to researchers was the easy availability of these tests on the Internet, for non-medical purposes. Companies now tout the test as a quick way for parents-to-be to kill their curiosity and find out the sex of their child as early as five to seven weeks' gestation.
"There are companies that say these tests can help determine the age of the child as early as five weeks,” said Dr. Bianchi, "We just didn't see that, but after seven weeks, the percentage of accuracy in determining a male was much higher and after 20 weeks, it shot up to more than 99 percent."
Because of the ease of the blood test, and the earlier window for sex determination, some medical ethicists feel the test makes it easier for parents to abort a child because of its sex. But scientists and policy experts say even though sex selection may have been documented sporadically in other parts of the world, there is no strong evidence of that happening in the U.S.
"There isn't really data on reasons people choose abortions," notes Susannah Baruch, a lawyer and policy consultant for "Generation Ahead," a U.S. organization that fights for policies that lead to more just and ethical uses of human genetic technologies. "There have been only a few small studies in Asian immigrant populations looking at birth rate ratios which show some skewing towards more boys in second and third born children. But none of it sheds much light on the way first semester screening would be used."
The blood tests are usually given to women who either have a relative or they themselves know they carry a genetic defect that could be transferred to their babies. In some cases, these defects or conditions can be reversed inside the womb, so doctors say the sooner parents know about their unborn baby's issues, the better. Although there is ultrasound technology, such as a sonography that can be performed as early as 11 weeks’ gestation to determine fetal sex, many parents may want to know sooner.
"The availability of reliable noninvasive alternatives to determine fetal sex would reduce unintended fetal losses," noted the authors, "And would presumably be welcomed by pregnant women carrying fetuses at risk for disorders.”
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