Prenatal Testing for Gender: How early should parents know their fetus’ sex?Heather Turgeon
Earlier this month, a study in the Journal of the American Medical Association reported that a simple blood test given to a pregnant woman at just seven weeks could predict the sex of her fetus. The technology, based on a procedure known as cell-free DNA testing, immediately stirred controversy, with some worrying that if moms and dads had such early and easy access to their baby’s pink or blue status, they might choose to abort fetuses of the “wrong” sex.
An early test for baby sex is just the tip of the iceberg, though. It’s part of a rapidly-advancing field that, like it or not, will change the landscape of prenatal testing in the near future – streamlining the complicated decision tree for a pregnant mom and serving up critical information about her growing baby just weeks after she finds out she’s pregnant.
To pinpoint sex, mom’s blood is scanned for bits of baby DNA. Scientists look for pieces of Y-chromosomes and, if they find them, it’s a boy. Lack of Y-chromosomes means the baby is XX, or girl. When it hits, the test will nudge the timeline for expecting couples, who would have the option to know the baby’s sex from a simple prick of mom’s finger early in the first trimester. (Right now, high-resolution ultrasound can hint at sex beginning around week 12, but it isn’t accurate until the second trimester. Chorionic Villus Sampling (CVS) gives an early definitive answer, but it’s an invasive procedure with slight risks.)
Some worry that with the new technology, parents who, let’s say, desperately wanted a girl, but find out through floating Y-chromosomes that they’re boy-bound, could terminate the pregnancy in a relatively hassle-free manner well before mom’s baby bump even starts to show.
But the truth is that testing for sex early on has real medical implications beyond that scenario. Knowing girl or boy status helps couples whose babies are at risk for sex-linked diseases like hemophilia and Duchenne muscular dystrophy, which affect males almost exclusively. If parents carrying genes for debilitating sex-linked disorders find out they’re having a girl at week seven, they can rest easy. If they’re having a boy, they can go forward with further testing.
And the debate over whether the test is a good idea is almost irrelevant because the technology is inevitable – and its application goes far beyond sex. Maternal blood tests will one day be able to give us all kinds of genetic information about a fetus without ever having to touch the baby itself or any of the surrounding fluids and tissues in the womb (parents currently have to sweat over the pros and cons of drawing samples by needle from baby’s home to get genetic answers). We’ve always known that bits of baby DNA escape through the placenta and make their way to mom’s bloodstream – in fact between 10-20 percent of the DNA molecules in mom’s plasma are actually from the fetus she’s carrying.
In the past, measuring those baby genes in a meaningful way has been difficult. But with recent advances, scientists can detect and measure millions of DNA fragments from mom’s blood in just days.
Which is why we’ve also heard recently about simple maternal blood tests for other conditions. Earlier this year, for example, scientists reported that they could tell if a baby had Down syndrome by testing mom’s blood. In this case, instead of looking for Y-chromosome pieces, the scientists look for extra bits of chromosome 21. Since three, instead of two, chromosome 21’s leads to Down syndrome, their greater proportion in the bloodstream indicates the baby has the disorder.
When these technologies make their way to the OB’s office, they’ll change the routines and choices for expecting couples – most likely replacing the current screening tests and shifting some couples away from amniocentesis and CVS. One day, sticking a needle through a woman’s belly to see her baby’s genes might seem like an archaic practice.
We’ll have to wait a bit for that scenario. None of these tests are available in the U.S. (although you can get sex tests by way of suspect internet-ordering), because they haven’t been approved by the FDA. And if terminating a pregnancy is on the line, a lot of couples will still want total accuracy. The sex-prediction test boasts 95 percent accuracy at seven weeks. The blood test for Down syndrome has been shown to pick up on all cases of the disorder but with two percent false positives – meaning that in two percent of cases in which the test read Down syndrome, the baby was actually fine. Impressive, but it still doesn’t top the accuracy of CVS or amnio.
Still, the new wave of genetic answers will make for some tough conversations between couples. In the end, though, the percentage of people in this country who would terminate a pregnancy based solely on sex is probably pretty low, and most who choose not to take a fetus with a chromosomal problem to term would likely stay with that choice whether it came at eight weeks or 12. The technology is coming, so there’s no reason to stoke fears – instead, doctors and genetic counselors should be talking about how to make the information and options clear and most helpful for parents.