How is second trimester screening done?

[Anonymous]

How is second trimester screening done, and how do I understand my quadruple marker result?

[Genetic Counselor]

The test needs only a sample of the mother’s blood. The laboratory measures four substances made by the baby and the placenta:

• Alpha-fetoprotein: a protein made by the baby. It is often lower than usual in Down syndrome and higher than usual when there is an open neural tube defect.
• Unconjugated estriol: A hormone made by the baby and placenta working together. It is often lower in Down syndrome and Edwards syndrome.
• Human chorionic gonadotropin: The main pregnancy hormone. It is often higher in Down syndrome and lower in Edwards syndrome.
• Inhibin A: A hormone made by the placenta. It is often higher in Down syndrome. The triple marker test leaves this one out, which makes it less accurate.

Each level is compared with the usual level for that exact week of pregnancy. The result is shown as a “multiple of the median”. A value of 1.0 is exactly average, 2.0 is twice the average, and 0.5 is half. A computer then combines these values with the mother’s age to give a chance for each condition, such as “1-in-3,000” or “1-in-120”. Each laboratory uses a cut-off, often around 1-in-250 or 1-in-270. A chance higher than the cut-off is reported as “high risk” or “screen positive”.

Getting the details right matters a great deal. Hormone levels change quickly week by week, so the pregnancy dates should be confirmed by ultrasound. A wrong date is one of the most common reasons for a misleading result. The laboratory also needs your weight, whether you have diabetes treated with insulin, whether you smoke, whether the pregnancy is from in vitro fertilization, and whether you are carrying twins.

The quadruple marker test finds about 81 of every 100 babies with Down syndrome, while flagging about 5 in every 100 pregnancies as high risk. It finds about 60 to 80 of every 100 babies with Edwards syndrome. The triple marker test finds about 69 in every 100 babies with Down syndrome. A low-risk result is reassuring, but it does not rule out a chromosomal condition completely.

[Author]

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