The performance of second trimester long bone ratios for Down syndrome screening is influenced by gestational age

Abstract

Objective: To determine if gestational age (GA) at the time of ultrasound impacts the positive predictive value of shortened femur and humerus lengths (FL, HL) for trisomy 21 (T21).

Methods: Sonograms from 14 to 21 and 6/7 weeks' gestation were collected over a 28 month period. Multiple gestations or fetuses with major structural anomalies were excluded. Biometric data and GA were obtained; the expected HL (or FL): observed HL (or FL) ratios were calculated using two regression formulas (Benacerraf and Nyberg). A HL ratio <0.90 and a FL ratio <0.91 were considered shortened. T21 fetuses were identified through database and chart review. Positive predictive values (PPV) for T21 of the shortened bone ratios were determined, then stratified by GA.

Results: Of the 2606 ultrasounds, 8.9% and 18.9% of fetuses had shortened HL and FL ratios, respectively, using the Benacerraf formula. Shortened ratios were noted significantly less commonly (2.3 and 4.4%, respectively, P < 0.001 for each) using the Nyberg formula. With either formula, abnormal bone ratios were more frequently documented with a GA less than 17 weeks (P < 0.001). There were 17 T21 pregnancies.

Conclusions: GA and formula selection influence the performance of long bone ratios as soft markers for T21 in the second trimester.