Maternal serum screening involves two tests during pregnancy to indicate the likelihood of fetal birth defects. The tests are non-invasive and may suggest further diagnostic tests are needed. The first test involves a blood test and ultrasound, while the second measures different hormones. The tests do not prove the existence of birth defects but suggest the amount of risk.
Maternal serum screening consists of two different tests performed on pregnant mothers at different times during pregnancy that can help indicate a higher or lower likelihood of certain fetal birth defects. These tests are not diagnoses of birth defects – they may indicate that further diagnostic tests are needed or that further diagnoses are probably not needed. Screenings are non-invasive and may be preferred over 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 week of pregnancy. The first item is a blood test from the mother, usually collected in a laboratory. The second part is an ultrasound.
Maternal blood is evaluated for levels of beta-HCG (human chorionic gonadotropin) and PAPP-A (pregnancy-associated plasma protein A). Variations in these levels from expected norms may indicate an increased risk of trisomy 18, Down syndrome and congenital heart defects. The blood results are compared to ultrasound measurements of the back of the fetus’ neck, which, if increased in thickness, may suggest additional risk.
If the results of a first trimester maternal serum screening suggested an additional risk of fetal disorders, women could have amniocentesis, chorionic villus sampling, or have a fetal echocardiogram a couple of months after the test. These additional diagnostic tests are needed because abnormal results on a maternal serum screen can be false alarms. A suggestion of an elevated risk of birth defects or genetic problems does not mean that a fetus is affected.
Some women have a first maternal serum screen with suggested abnormal results and have a second test during the second trimester. Alternatively, women may wait until their second trimester before getting a screening, or they may not have any at all. The second screening is a triple test that measures different things. Look for hcG, but also look at estradiol and alpha-fetoprotein. A quad test, an alternative, measures another hormone in the mother’s blood, called inhibin-A, increasing the accuracy of the test readings by about 10%-15%.
Second trimester maternal serum screening also evaluates other factors when considering the risk of having a baby with certain birth defects or genetic problems. The number of previous births, maternal age, and maternal health are part of creating a ratio that suggests risk. Again, these tests do not prove the existence of birth defects. They only suggest the amount of risk based on the test results. On the other hand, consistent first and second trimester test results can quite reliably predict conditions like Down about 90-95% of the time.
Maternal serum screening is not required, and some women may report screening horror stories that inaccurately suggested birth defects. Some expectant mothers like the option of having these tests because they may rule out the need for a broader diagnosis, although even tests with normal results can be inaccurate. As more is learned, there is evidence that these tests are actually more accurate than supposed, and some scientists suggest that further refinements of such screenings could eventually render more invasive diagnostic tools useless.
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