Influence of maternal BMI on the role of second trimester genetic sonography in the faster trial


      The FASTER trial has previously demonstrated likelihood ratios (LR) for soft markers on 2nd trimester genetic sonography may be used to improve the performance of 1st or 2nd trimester screening. We hypothesized that this capacity for risk modification may be limited in obese women and sought to evaluate the influence of maternal BMI on the ability to detect multiple 2nd trimester markers of fetal aneuploidy.

      Study design

      Unselected singleton pregnancies with prior screening who remained at a FASTER site for antenatal care underwent detailed genetic sonogram at 14 to 24 wks to evaluate for major structural fetal anomalies and minor markers for aneuploidy. Body-mass indices (kg/m2) were calculated from reported values at the initial visit, and women were stratified into BMI categories: <19 underweight; 20-24.9 normal; 25-29.9 overweight; 30-34.9 class I obesity; 35-39.9 class II; >40 class III. Sensitivity, specificity, FPR, FNR, and LR, and a missed diagnosis rate [MDR: (FN + marker recorded as “missing”)/N] were calculated.


      8,555 patients with complete BMI information had detailed 2nd trimester genetic sonography. While no cases of Trisomy 21 were missed by the overall screening program, poorer performance of genetic sonography to identify affected fetuses occurred in obese when compared with non-obese (BMI<25) women. 3 or more markers (sensitivity 0% vs 5.4%, LR 0.2 vs 34), absence of observed markers (sensitivity 22% vs 38%, FPR 91% vs 88%), nuchal fold >5mm (MDR 50% vs 38%), and echogenic foci (MDR 2.6% vs <1%) all were less predictive among obese women. A significant inability to obtain 1st trimester NT measurements in obese women was also observed (BMI<25 1.0%; 25 to<30 1.4%; >30 3.2% with class III 7.8%, p<.0001).


      While the use of likelihood ratios from 2nd trimester genetic sonography improves screening performance, the ability to detect multiple minor markers of aneuploidy is influenced by maternal BMI. These data emphasize the relative importance of maternal serum screening in the detection of aneuploid fetuses among obese women.