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Maternal Serum Screening

Down Syndrome, Trisomy 18 and Neural Tube Defects

Second Trimester Screening

Slide 12

March 2011

Second trimester screening is performed between 15 and 22 weeks of gestation.

The biochemical markers used to derive a risk estimate are: alpha-fetoprotein or AFP, unconjugated estriol or ue3, hCG and inhibin A. AFP is used on the risk calculation of the all 3 conditions. Low levels of AFP are seen in Down syndrome and trisomy18 pregnancies, whereas increases are seen in neural tube defects. Unconjugated estriol, an estrogen produced by the placenta, is decreased in the majority of Down syndrome or trisomy 18 pregnancies. hCG is normally increased in Down syndrome pregnancies and decreased in trisomy 18 pregnancies. Inhibin A is only used to calculate the risk for Down syndrome, as in these cases it tends to be elevated.

Second Trimester Screening


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