From the Guidelines
Maternal Serum Screening (MSS) is a crucial tool for detecting common birth defects, such as Neural Tube Defects (NTDs) and Trisomies, and should be performed between 15-22 weeks of pregnancy. The primary purpose of MSS is to identify women at risk for fetal open neural tube defects (ONTDs), anencephaly, and trisomies 21 and 18, as stated in the study by 1.
Key Components of MSS
- The standard screening includes a quad screen that measures four substances in the mother's blood: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), estriol, and inhibin A, as discussed in 1.
- This screening helps identify risk for neural tube defects like spina bifida, chromosomal abnormalities like Down syndrome (trisomy 21), trisomy 18, and certain other birth defects.
- The optimal time for NTD screening is at 16 to 18 weeks gestation, and testing can be done between 15 and 20.9 weeks, with the laboratory informed of the gestational age at the time the sample was drawn for an accurate interpretation, as noted in 1.
Importance of MSS
- MSS is recommended for all pregnant women to detect common birth defects, and patients should be counseled that screening is optional, with results interpreted by healthcare providers considering maternal age, family history, and other risk factors.
- The screening's accuracy varies, with detection rates of 80-90% for Down syndrome and 80-85% for neural tube defects, with false positive rates of 5%, as mentioned in 1.
- Abnormal results require follow-up with diagnostic tests like amniocentesis or chorionic villus sampling, highlighting the importance of MSS as a screening tool rather than a diagnostic test.
Factors Affecting MSS Accuracy
- Maternal weight, race, presence of insulin-dependent diabetes, number of fetuses, and family history of ONTD are factors used to adjust the multiples of the median (MoM) level, as discussed in 1.
- Ultrasound dating of the pregnancy reduces the false-positive rate and increases the detection rate of ONTDs, emphasizing the importance of accurate gestational age assessment.
Clinical Guidelines
- Cut-off levels for ONTD screening are 2.0 to 2.5 MoM in singleton pregnancies, and 4.0 to 5.0 MoM in twin pregnancies, as stated in 1.
- Genetic counseling and additional testing such as targeted ultrasound examination and amniocentesis are recommended for pregnancies with an elevated MSAFP test result, highlighting the need for comprehensive follow-up care.
From the Research
Purpose of Maternal Serum Screening (MSS)
The purpose of Maternal Serum Screening (MSS) is to detect fetuses with an increased risk for common birth defects, such as:
- Neural Tube Defects (NTDs) [(2,3,4,5)]
- Trisomies, including Down syndrome and Trisomy 18 [(2,3,4,5,6)]
Screening Process
MSS measures various serum markers, including:
- Alpha-fetoprotein (AFP) [(3,4,5,6)]
- Human chorionic gonadotropin (hCG) [(3,4,5,6)]
- Unconjugated estriol (uE3) [(3,4,5,6)] These markers are used to calculate the risk of fetal Down syndrome or open neural tube defect 3.
Importance of MSS
MSS is a non-invasive means of identifying pregnant women at an increased risk for various conditions, including fetal chromosome abnormalities 5. The screening test can detect a significant portion of neural tube defects and trisomy 21-affected pregnancies 4. However, it is essential to counsel patients about the risks and benefits of MSS to provide a balanced discussion of screening issues 4.