Negative results mean that the risk is less than the established cutoff they do not guarantee the absence of Down syndrome. Second-trimester results are negative when the calculated risk is below 1/270 (0.37%). If part 1 is negative, submit an additional specimen in the second trimester (order SEQB / Sequential Maternal Screening, Part 2, Serum). A positive result indicates that the estimated risk exceeds the screen cutoff.įirst-trimester results are negative when the calculated risk is below 1/50 (2%). A negative result indicates that the estimated risk falls below the screen cutoff. Maternal screens provide an estimation of risk, not a diagnosis. Low PAPP-A levels before the fourteenth week of gestation are associated with an increased risk for Down syndrome and trisomy 18. Levels in maternal serum increase throughout pregnancy. PAPP-A is highly expressed in first-trimester trophoblasts, participating in regulation of fetal growth. PAPP-A is a metalloproteinase that cleaves insulin-like growth factor-binding protein-4 (IGFBP-4), dramatically reducing IGFBP-4 affinity for IGF1 and IGF2, thereby regulating the availability of these growth factors at the tissue level. PAPP-A is a 187-kDa protein comprised of 4 subunits: 2 PAPP-A subunits and 2 pro-major basic protein subunits. Increased fetal NT measurements can therefore serve as an indicator of an increased risk for Down syndrome and trisomy 18. While fetal NT measurements obtained by ultrasonography increase in normal pregnancies with advancing gestational age, Down syndrome and trisomy 18 fetuses have larger NT measurements than gestational age-matched normal fetuses. The NT measurement, an ultrasound marker, is obtained by measuring the fluid-filled space within the nuchal region (back of the neck) of the fetus. If results are positive, the patient is typically offered counseling and diagnostic testing. The information from both trimesters is combined and a report is issued. The blood specimen is tested for AFP, unconjugated estriol, human chorionic gonadotropin, and inhibin A. If the risk from the first trimester is below the established cutoff, an additional serum specimen is collected in the second trimester for SEQB / Sequential Maternal Screen, Part 2, Serum. For a stand-alone NTD-risk assessment, order MAFP1 / Alpha-Fetoprotein (AFP), Single Marker Screen, Maternal, Serum. When the part 1 screen is completed, NTD risk is not provided. In that event, the patient is typically offered counseling and diagnostic testing. If the result from part 1 indicates a risk for Down syndrome that is higher than the screen cutoff, the screen is completed, and a report is issued. The results of the ultrasound measurement and blood work, along with the maternal age and demographic information, are used to calculate trisomy 21 (Down syndrome) and trisomy 18 risk estimates. Along with the NT measurement, a maternal serum specimen is collected to measure pregnancy-associated plasma protein A (PAPP-A). Therefore, NT data is accepted only from NT-certified sonographers. The ultrasound measurement, referred to as the NT measurement, is difficult to perform accurately. This test involves an ultrasound and a blood draw. Sequential screening combines biochemical and ultrasound markers (nuchal translucency: NT) measured in both trimesters of the pregnancy. Sequential screening is a type of cross-trimester screening that has an improved detection rate as compared to either first- or second-trimester screening. Various options for maternal serum screening are available and include: first trimester, second trimester, and cross-trimester. Maternal serum screening is used to identify pregnancies that may have an increased risk for certain birth defects, such as trisomy 21 (Down syndrome), neural tube defects (NTD) and trisomy 18.
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