Next-generation sequencing technologies enable early, non-invasive prenatal testing
As women wait longer to get married and start a family more expectant moms may find themselves booked for a visit with a genetic counselor. Though the words “genetic counselor” may sound scary, perhaps conjuring images of a futuristic society that’s embraced eugenics, you can relax.
All pregnant women who are 35 and over are advised to meet with a genetic counselor before their first ultrasound to review family history, discuss chances of genetic disorders and explore genetic testing options. It's truly routine for women of advanced maternal age to be sent to counseling.
“We ask you about yourself and we decide together what your best options are,” explained Lauren Korty, lead genetic counselor at UC San Diego Health System’s Fetal Care and Genetics Center
, which recently relocated to Sorrento Valley. “We are trained to be non-directive and to help reduce anxiety.”
Another reason many women may find genetic counseling a stress reliever is that it is now possible to screen for some of the most common genetic abnormalities earlier, more accurately and without any risk to the fetus.
These non-invasive prenatal tests became available in late 2011 and have since led to a 30 percent reduction in traditional amniocentesis and chorionic villus sampling (CVS), both of which involve inserting either needles or catheters into the womb and carry an increased risk of miscarriage.
The pluses of the new screening method include: a greater than 98 percent detection rate for Down’s syndrome (the most common chromosomal abnormality, affecting 1 in 700 live births) and a less than 0.5 percent false positive rate. Traditional early screening tests have 80 to 90 percent detection rates and 3 to 5 percent false positive rates.
Non-invasive testing can also be done as early as the tenth week of pregnancy, with results available within a week. Last but certainly not least, testing poses no risk to the fetus.
“Before non-invasive testing, the only way women with an increased risk of chromosome abnormalities on routine serum screening could get more information was to proceed with an invasive test that had a 1 in 300 to 1 in 500 chance of causing a miscarriage,” Korty said. “Now, we can offer this person a test that has no risk to their pregnancy.”
Non-invasive prenatal testing is based on a technology known as massively parallel genomic sequencing, which uses a highly sensitive assay to quantify millions of DNA fragments in biological samples.
In the context of fetal genetics testing, the assay is tuned to detect fetal DNA fragments in the mother’s blood plasma. Fetal DNA is believed to enter the maternal bloodstream via the placenta.
Higher than expected amounts of DNA associated with a particular chromosome indicate the presence of three copies of that chromosome instead of the usual pair, a condition known as trisomy, caused by errors in cell division of either sperm or egg cells.
Trisomy 21 (three copies of the 21st chromosome) is the cause of about 95 percent of all cases of Down's syndrome, while trisomy 13 and trisomy 18 result in rare but fatal birth defects.
The test also detects lower than expected amounts of chromosomal DNA (monosomy), associated with conditions such as Turner syndrome.
“Non-invasive prenatal testing is what everyone is most excited about right now,” Korty said. “It’s not a replacement for invasive diagnostic procedures, but it is reducing unnecessary invasive procedures and it will continue to evolve.”
Korty recommends fetal DNA testing for all women 35 and over, women with increased risk of a chromosome abnormality on routine serum screening, pregnancies with ultrasound findings or for families with a previous trisomy.
Fetal DNA testing is not recommended for low-risk women or those with multiple gestations because it has not been sufficiently evaluated in these groups, according to the American College of Obstetricians and Gynecologists.
Fetal Care and Genetics