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Maternal Serum Screening

Down Syndrome, Trisomy 18 and Neural Tube Defects

Maternal Serum Screening: Risk Calculation

Slide 5

March 2011

For all 3 screening options, the laboratory uses a mathematical model to calculate a woman risk of having a baby with Down syndrome, trisomy 18 or neural tube defect. This mathematical model takes into consideration the maternal age, the serum levels of various biochemical markers and, in the case of first trimester and sequential screening, the fetus ultrasound measurements.

In addition, a number of factors play a key role in the calculation of the risk as they will affect the values of the analytes. This includes gestational dating, weight, race, diabetic status of the individual, the number of fetuses present, and whether IVF treatment was used for conceiving.

For example, alpha-fetoprotein or AFP, one of the markers used in the Down syndrome and trisomy 18 risk calculation, is affected by some of these factors. Black mothers tend to have higher AFP levels but lower risk of neural tube defects and are assigned to a separate AFP median set. The AFP is adjusted upward in insulin-dependent diabetic mothers to account for lower values in diabetic pregnancies. AFP multiple of the median (MOM) values are adjusted in heavier mothers to account for the dilution effect of the analytes.

Inaccurate information can lead to significant alterations in the estimated risk. This is why it is so important that accurate information is provided when the sample is submitted for analysis.

Risk Calculation


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