Maternal serum screening The Genetics Center, Inc.

The Genetics Center, Inc. Address, phone, and fax

Genetics and Maternal Serum Screening:

Some straightforward answers to
common questions

Maternal serum screening is designed to identify pregnant women who could benefit from prenatal testing for Down syndrome, spina bifida, and certain other birth defects. These tests are done on a blood sample and pose no risk to the developing baby.

Why screen all pregnant women?

During a pregnancy, every couple is concerned about whether their baby will be healthy. Fortunately, over 95 percent of babies will not have any significant health or learning problems. Two relatively common disorders, Down syndrome and spina bifida, most often occur where there is no family history of these disorders. Since they are both serious conditions that can be prenatally diagnosed in the early stages of pregnancy, screening can be very helpful for any pregnant woman.

Why focus on Down syndrome and spina bifida?

Screening for Down syndrome and spina bifida, unlike many other genetic conditions and birth defects, can be done using a blood sample from the mother at 15 to 17 weeks gestation. While not very frequent, Down syndrome and spina bifida are not rare. Down syndrome, a condition in which significant mental retardation is present, occurs in approximately 1 in 650 births. Spina bifida, which occurs in about 1 in 750 births, is usually associated with serious medical and neurologic problems. Many parents are interested in knowing whether or not their baby will have either condition.

Alpha-fetoprotein (AFP) is measured in maternal blood, and an elevation indicates an increased risk of a baby having spina bifida. A low AFP value increases the risk that the baby has Down syndrome. Beta-hCG (the hormone measured in blood and urine pregnancy tests) is elevated in pregnancies where the fetus has Down syndrome. Other substances present in maternal blood can also be measured and help in estimating the risk of having a baby with Down syndrome. Multiple marker serum screening involves measuring AFP and one or more additional substances. When three substances are measured, it is sometimes referred to as a triple screen, and a quad screen involves measuring AFP, beta-hCG, unconjugated estriol, and inhibin-A. Obstetricians typically offer multiple marker serum screening to all pregnant women who will be less than 35 years old at delivery. In some circumstances, it may be beneficial for other women as well.

How does a screening test differ from a diagnostic test?

Screening tests are designed to identify those individuals who might benefit from diagnostic testing. Diagnostic tests are intended to be highly accurate and to correctly predict whether or not there is a problem. Since most couples are concerned about even the small possibility of having a baby with a birth defect that can be diagnosed prenatally, the availability of easy to perform and sensitive screening tests can be reassuring.

When is serum screening performed?

Multiple marker screening is typically performed at 15-17 weeks gestation, although it can be performed at 14-22 weeks gestation, if necessary. Specialized serum screening in the first trimester, at 10-12 weeks gestation, in conjunction with ultrasound measurements of the fetal nuchal translucency, is performed in some centers and permits earlier prenatal diagnosis.

What if screening tests are normal?

Normal results of the screening, which is the case over 90 percent of the time, should be reassuring. This typically means that no further testing for Down syndrome or spina bifida will be necessary. Unfortunately, screening tests for Down syndrome and spina bifida do not always identify an affected fetus. It should be comforting to know, however, that maternal serum screening detects over 85 percent of fetuses with spina bifida, and multiple marker screening identifies 60-75 percent of fetuses with Down syndrome.

The decision to have maternal serum screening is a personal choice. Obstetricians usually recommend that screening be considered, and they are able to answer questions about these studies. It is up to each individual or couple to determine whether maternal serum screening is appropriate for them.

What if the screening testing is abnormal?

Most women who have abnormal screening results have normal babies. Remember, the results of the screening test are only used to determine if further testing is recommended.
If the screening results are abnormal, further testing is available, but it will be necessary to decide what testing is indicated and to determine whether the couple wishes to pursue this testing. Although maternal serum screening and ultrasound examinations pose no risk to a developing baby, amniocentesis, which is often necessary for definitive diagnostic testing, carries a small (0.5 percent) risk of miscarriage.
If an abnormal screening result is obtained, a couple may be asked to consult a geneticist, who will obtain a detailed family and pregnancy history, review the results of the screening tests, and help the couple and the obstetrician decide how to proceed with further testing.