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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Prenatal Genetic Screening – Serum and Ultrasound
Sponsored by The Indian Health Service Clinical Support Center
6. Down syndrome
For a multiple marker screening (MMS) reported as positive for Down syndrome
(DS), about 3-4% will be true positives. Accurate gestational dating, maternal age, weight, diabetic status, and race, all factor in importantly. Of note, specific normative data on Native American women are currently not available. It is also important to know if your lab uses a cut-off of 1:270 for a positive screen for DS (the risk of a 35 y/o woman) or the lower cut-off of 1:190, the more specific, but less sensitive, number. Again, ultrasound is important to be sure dates are correct. If the ultrasound indicates a date more than 10 days different from the menstrual dates, the laboratory should be contacted to recalculate the results with the correct information. Unlike NTD screening however, abnormal MMS suggesting a higher risk of fetal DS should NOT be repeated. Patients should be triaged to the next step, high resolution ultrasound and/or amniocentesis.
Because of societal demographics, live births to women 35-49 years of age had increased to over 12% in the year 2000. This has resulted in almost half of the DS fetuses in the second trimester being carried by older women (a high rate of spontaneous loss of aneuploid fetuses occurs prior to term). MMS is thus a cost-effective way to initially evaluate these patients, and avoid an excessive number of amniocenteses. As noted above, amniocentesis has a 0.5% (1 in 200) procedure related pregnancy loss rate, a very important consideration for many women.
In Practice Bulletin Number 27, ACOG issued this Level B recommendation “ Counseling for amniocentesis in a twin pregnancy in women age 33 years is indicated because the midtrimester risk of fetal Down syndrome is approximately the same as for that of a singleton pregnancy at age 35 years.” Considering the information above the ACOG recommendation should be considered as part of a staged diagnostic scheme may be an option to discuss with your older pregnant patient.
