Maternal serum screening consists of two different tests performed on pregnant mothers at different times during pregnancy that may help indicate greater or less likelihood of certain fetal birth defects. These tests are not diagnosis of birth defects — they could indicate the need for additional diagnostic testing or that further diagnosis is probably unnecessary. Screenings are non-invasive and may be preferred to starting out with more extensive and, in some cases, riskier diagnostic methods.
The first of the maternal serum screening tests has two elements and takes place between the 11th and 13th weeks of pregnancy. The first element is a blood test of the mother, usually collected in a lab. The second part is an ultrasound.
Maternal blood is evaluated for beta-HCG (human chorionic gonadotropin) levels and for PAPP-A (pregnancy associated plasma protein-A). Changes in these levels from expected norms may indicate increased risk for trisomy 18, Down syndrome, and congenital heart defects. Blood results are compared to ultrasound measurements of the back of the fetus’ neck, which, if increased in thickness, may suggest additional risk.
Should findings from a first trimester maternal serum screening suggest additional risk for fetal disorders, women could undergo amniocenteses, chorionic villus sampling or have a fetal echocardiogram a couple of months after testing. These additional diagnostic tests are needed because abnormal findings on a maternal serum screening can be false alarms. A suggestion of elevated risk for birth defects or genetic problems doesn’t mean a fetus is affected.
Some women have a first maternal serum screening with suggested abnormal findings and have a second test during the second trimester. Alternately, women may wait until the second trimester prior to having a screening, or they may not have one at all. The second screening is a triple test that measures different things. It looks at hcG, but also evaluates estradiol, and alpha-fetoprotein. A quad test, an alternative, measures one more hormone in maternal blood, called inhibin-A, increasing accuracy of test readings by about 10%-15%.
The second trimester maternal serum screening also evaluates other factors when considering risk for having a baby with certain birth defects or genetic issues. Number of previous births, maternal age, and maternal health are part of creating a ratio that suggests risk. Again, these tests do not prove birth defects exist. They only suggest risk amount based on test findings. On the other hand, consistent findings of first and second trimester testing may pretty reliably predict conditions like Down’s about 90-95% of the time.
Maternal serum screening is not required, and some women can relate horror stories of screenings that inaccurately suggested birth defects. Some expectant moms do like the option of having these tests because they may rule out need for more extensive diagnosis, though tests with normal findings may also be inaccurate. As more is learned, there is evidence that these tests are actually more accurate than supposed, and some scientists suggest greater refinements of such screenings may eventually render more invasive diagnostic tools unnecessary.