The older mothers-to-be are, the higher their risk of having babies with this fairly common birth defect. But nothing suddenly changes at the 35th birthday, and a 20-something can have a baby with Down syndrome, too.
Now growing evidence suggests that offering blood tests to screen every pregnant woman for the risk may be more effective than using today's standard age cutoffs for amniocentesis — decreasing unnecessary amnios by culling out which women of any age most need the invasive test, which is more accurate but occasionally causes miscarriage.
"Twenty years ago, the only way to identify women at risk was to offer amnio to those older than 35. That makes no sense any longer," says Dr. Rebecca Smith-Bindman of the University of California, San Francisco, who co-authored a large new study that supports screening-based instead of age-based decisions.
It's a policy that California has adopted, with a unique program that provides free amniocentesis for women whose screening tests determine they're at high risk.
Whether to have an amnio is an agonizing decision that more Americans are making as they increasingly postpone childbearing into their 30s and even 40s. For those who wouldn't consider abortion, prenatal diagnosis still is important because babies with Down syndrome — in which an extra chromosome causes mental retardation and other defects, often including heart problems — can need specialized care at delivery that affects hospital choice.
But women have a 1 percent chance of suffering a miscarriage from amniocentesis, the use of a needle to draw fluid from the amniotic sac, or from a similar invasive test called chorionic villus sampling. While it's a small risk, some women won't chance a test that could cost them a healthy baby.
Additionally, "one of the misconceptions in the general public is if you're not over 35, you don't have to worry about Down syndrome," says Dr. Xavier Pombar of Rush University Medical Center in Chicago.
While risk increases with each year of maternal age — from one in 1,200 at age 25 to about one in 300 at 35 — most Down syndrome babies are born to women under 35 simply because younger women have more babies.
Hence, women under 35 are supposed to be offered certain blood tests — most common is the "triple-screen" — that can identify about three-quarters of Down syndrome cases by measuring different chemicals in the mother's blood. Missed cases aside, blood tests also can cause false alarms, so a positive blood test requires a definitive amnio to double-check.
The new study suggests that blood tests, or specialized ultrasound exams that also can spot defects, should be the amnio deciding factor even for older moms.
Smith-Bindman and British colleagues examined 6 million births over the last decade in England and Wales, where different regions use either age or blood/ultrasound screening to determine who needs an amnio.
In areas where women got amnios based on screening, 50 percent more Down syndrome cases were diagnosed prenatally, with half as many invasive tests performed, compared with areas where age determined testing, concludes the study in last month's American Journal of Obstetrics and Gynecology.
"This is a good way to go," says Pombar, who says some U.S. hospitals are finding similar results. "You're doing less amniocentesis ... and losing less normal babies."
Screening is cutting amnios in California, too, says Dr. George Cunningham. A state program offers all pregnant women the triple-screen, done between weeks 15 and 20 of pregnancy, for $105. Those whose blood tests indicate they're at high risk then can get genetic counseling and an amnio for free. (An amnio usually costs about $1,500; insurance coverage varies.) Women who want to skip straight to an amnio because of age or birth defects in the family can opt out of the program.
There is a trade-off: A 35-year-old could forego amnio in favor of a blood test that offered false reassurance, notes Dr. Serdar Ural of the University of Pennsylvania, who stresses that age-spurred amnio remains the U.S. standard of care.
But some doctors estimate that only a quarter to half of over-35 women agree to amniocentesis anyway.
At the least, "if you were on the fence about amnio, you would want to have an ultrasound and certainly want to consider serum testing," says genetics expert Peter Benn of the University of Connecticut.
By Lauran Neergaard