Prenatal testing for pregnancy has evolved greatly over the last few decades. Ultrasounds have leaped into the 21st century with exquisite 3-dimensional pictures and sophisticated blood tests which allow both parents and physicians to adequately prepare for many possible infant outcomes. If an infant is to be born with a particular medical need, it can be invaluable to know in advance. The most common prenatal tests include a battery of laboratory tests, ultrasonic imaging, and sometimes more invasive procedures such as amniocentesis and chorionic villus sampling or CVS.
Screening for infants with three types of the most common genetic disorders including trisomy 13, 18 and 21 has just become easier with the advent of a new genetic maternal blood test. Down syndrome in infants is caused by trisomy 21, which occurs when the baby has three copies of chromosome 21 instead of two copies. Children born with Down syndrome have several physical and cognitive impairments including heart defects, hypothyroidism, and respiratory problems, as well as intellectual disabilities or mental retardation.
A very recent blood test has been developed to allow healthcare providers to administer a maternal blood test at around 10 weeks of pregnancy to women with a higher risk of carry a trisomy 21 infant. These women include mothers over the age of 35 and women who have had a previous child with trisomy 21. Maternal age seems to be a key factor as incidences of trisomy 21 increase to 1 in 350 with mothers over 35 and 1 in 35 with mothers over 45. The rate of incidence decreases to as low as one in 1470 for women at the age of 20. This blood test measures cell free fetal DNA or cffDNA and measures extra sequences of number 21 chromosome. Generally, results can take from 1 to 2 weeks. This test is not a definitive diagnostic test but strongly suggests a risk of having a child with trisomy 21 with very good accuracy. Some tests claim a false positive rate of less than 0.1 percent.
A positive test result will allow healthcare providers to offer more conclusive testing such as amniocentesis or chorionic villus sampling or CVS. Both amniocentesis and CVS are invasive procedures that carry a slight risk of miscarriage. Presently all high-risk pregnant women are offered the traditional method of prenatal testing for Down syndrome, which includes a first trimester blood test to measure pregnancy-associated plasma protein A or PAPP-A, as well as the hormone HCG or human chorionic gonadotropin. These blood tests are coupled with an ultrasound that measures the size of a particular region of the fetus’ neck known as a nuchal translucency test.
All women are also offered the second semester quad screening test which measures four separate substances present during pregnancy to include estriol, HCG, inhibin A, and alpha fetoprotein. A low level of alpha-fetoprotein can be an indicator of trisomy 21 and requires the more invasive amniocentesis or CVS screening to confirm Down syndrome.
When any of these methods come back with levels that may suggest trisomy 21, women are offered the opportunity to have the more invasive tests done to confirm the likelihood they are carrying a child with trisomy 21. The benefit of the new maternal blood test is that its accuracy minimizes the need for many women to have needless invasive tests that could possibly result in miscarriage. High-risk pregnant women should talk to their healthcare providers about the advances in prenatal testing and find out if testing would be right for them.
Dr. Hardy is a solo physician at Atlantic Ob/Gyn with locations in Chesapeake and Virginia Beach. For more information, please call 463-1234 or visit www.atlanticobgyn.com.