DIAGNOSTIC AND SCREENING TESTS
Prenatal testing is available for most important birth defects. When optimally performed and interpreted by experts, ultrasound can both "screen" for a variety of major defects and can also make a specific diagnosis. For other types of defects - particularly Down syndrome - screening tests such as ultrasound may not be diagnostic themselves but can help identify those patients at highest risk who might consider a diagnostic test. Diagnostic tests can tell with certain whether a fetus has Down syndrome with certainty but provide limited information about other types of birth defects.
The two diagnostic tests for Down syndrome and other chromosome abnormalities are:
These diagnostic tests can tell whether a fetus has Down syndrome with nearly 100% accuracy. However, this certainty comes with a potential price. These tests are “invasive” and require sampling either the amniotic fluid or the placenta with a needle. Although the risk is small, approximately 1 in 200 babies will be lost as a result of genetic amniocentesis and 1 in 100 will be lost as a result of CVS. Also, neither test can test for the majority of birth defects associated with normal chromosomes.
Screening tests use maternal age, fetal ultrasound, and/or maternal blood proteins to identify those patients at highest risk for fetal Down syndrome and who could benefit from a diagnostic invasive test.
The oldest screening
"test" is maternal age, based on the fact that the risk of major
chromosome abnormalities increase with age. A rather arbitrary cutoff
of 35 years is commonly used. Maternal age alone is not an effective
screening test since it results in detection of less than half of all
fetuses with Down syndrome, and many normal fetuses undergo needless
invasive testing. However, maternal age can be effectively combined
with ultrasound and/or biochemistry . Also, women aged 35 or older have
the option of a diagnostic test (genetic amniocentesis) based on age
alone. To see the risk of fetal Down syndrome based on maternal age
Combined First Trimester Screen for Fetal Aneuploidy (10-14 weeks) A new screening test, the Combined First Trimester Screen for Fetal Aneuploidy, has a detection rate of 80-90% for fetal Down syndrome. It can also help to identify fetuses at an increased risk of other types of birth defects, such as heart defects. This test can be performed earlier in pregnancy (10-14 weeks) than other screening methods. This test combines measurements of two hormones (free beta HCG and PAPPA) from the maternal blood, together with the maternal age and ultrasound measurements (nuchal translucency and crown-rump length) to calculate an overall risk for fetal Down syndrome as well as trisomy 18.
Maternal biochemistry (15-20 weeks).
The first protein
routinely measured in the maternal blood was AFP (alpha-fetoprotein).
High AFP levels were originally used to identify fetuses at risk for
neural tube defects. In 1984 it was discovered that low AFP levels may
help identify fetuses with Down syndrome. The blood test, sometimes
still called the "AFP test", evolved by 1990 into the so-called “triple
screen”, since it tested 2 other blood proteins. More recently the
maternal blood screen has evolved nto the “quad screen” because a fourth
marker was also found to be useful. These tests are usually performed
at 15-20 weeks of pregnancy by taking a blood sample at your doctors
Genetic Sonogram or Detailed Fetal Survey (15-20 weeks). During the 1980s and 1990s, improvements in prenatal ultrasound developed in parallel with maternal biochemistry. Second trimester ultrasound can look for a variety of subtle differences between normal fetuses and those with Down syndrome as part of a so-called “genetic sonogram”. A carefully performed genetic sonogram can detect 50-70% or perhaps even more of cases of fetal Down syndrome when performed at experienced centers. Even more importantly, a carefully performed second trimester sonogram can detect the majority of important birth defects associated with normal chromosomes and so would not be detected by amniocentesis. For example, ultrasound can detect nearly all neural tube defects whereas AFP screening detects 80-90% of neural tube defects. The detailed fetal anatomic survey might be considered the "baby's first physical exam" because it is performed much like a physical exam, from "head to toe". It can actually detect some types of abnormalities that would not be suspected by the usual physical exam after birth, although many other birth defects- usually minor- will be detected by the physical exam after birth but not the ultrasound survey.