In 2007, the medical guidelines changed from only offering women 35 years old and older prenatal testing for Down syndrome to offering it to all expectant mothers. This, however, should not mean maternal age is irrelevant when considering prenatal testing. And, it turns out, something commonly reported about maternal age is incorrect.
When the American Congress of Obstetricians and Gynecologists (ACOG) changed the guidelines in 2007, it did so at the prompting of the then new prenatal test of nuchal translucency (NT). When combined with a test of certain components of the mother’s blood, NT reported comparable accuracy rates as existing second-trimester blood-based tests (namely the AFP and then the Quad test). NT, though, is performed in the first trimester. So, the earlier and just-as-accurate prenatal test justified offering prenatal testing for Down syndrome to all expectant mothers.
Another reason for offering prenatal testing to all expectant mothers was the assertion that most children with Down syndrome are born to women under the age of 35. Indeed, this is exactly what the Centers for Disease Control states:
The age of the mother is the only factor that has been shown to increase the risk of having a baby with Down syndrome. This risk increases with every year, especially after the mother is 35 years of age. However, because younger women are more likely to have babies than older women, 80% of babies with Down syndrome are born to women younger than 35 years of age.
This statistic–that 80% of all children born with Down syndrome are born to women under 35–has been cited by the March of Dimes and the National Coalition for Health Professional Education in Genetics (NCHPEG). And, it turns out, it is wrong.
ACOG did not just change its guidelines overnight. Instead, practitioners argued in professional journal articles that maternal age should not set an arbitrary cut-off rate for offering prenatal testing. And, with the advent of the then-new first trimester ultrasound test of NT, ACOG changed its guidelines. But, a point may have gotten lost in the change.
In response to one of the more popular articles arguing for dropping the age cut-off, Robert Resta, a genetic counselor in Seattle, Washington and former editor of the Journal of Genetic Counseling, wrote a letter making a key point.
This statement is incorrect. The percentage of pregnancies in women aged 35 years or older has tripled since 1975 and currently about 14% of births in the United States are to women aged 35 and older. The net result is that since about the year 2000, women aged 35 and older have accounted for slightly more than 50% of pregnancies with Down syndrome.
Rather than dispute Resta’s assertion, the authors who had argued for offering prenatal testing for Down syndrome to all women because most children with Down syndrome are born to mothers under the age of 35 reversed themselves:
Data from Dr. Resta’s article indicate that he is correct in stating that the increasing number of pregnancies occurring in women who are 35 years of age and older has recently resulted in that cohort accounting for slightly more than half the number of Down syndrome fetuses conceived in the United States. … In the FASTER trial, where 21.6% of the screened population was aged 35 or older, compared with 14% for the US population, 69% of the Down syndrome cases occurred among those older women.
The FASTER trial was the First and Second Trimester Evaluation of Risk trial that established the reliability of NT as a prenatal test and was the prompt for ACOG to change its guidelines.
In a previous post, I shared how, due to The Great Recession, overall births are at their lowest point but have remained the same or growing among mothers in their 30’s and 40’s. This, then, means that there is an even higher percentage of older mothers being pregnant and, ergo, even more cases of Down syndrome are due to pregnancies of women over the age of 35. Contrary to the CDC statement, women under the age of 35 account for a minority of Down syndrome pregnancies, and have since the start of this century.