ULTRASOUND DETECTION OF FETAL TRISOMY-18 AND TRISOMY-21 BY 2ND-TRIMESTER GROWTH-CURVES

Objectives: The purpose of this study was to establish second trimeste r fetal growth curves based on ultrasound estimated fetal weight and t o determine their sensitivity to detect trisomies 18 and 21. Methods: Computerized ultrasound reports for 3,119 patients between 12 and 24 w eeks' gestation were retrospectively reviewed. Patients with a reliabl e last menstrual period confirmed by ultrasound (+/-2 weeks) and no fe tal anomalies were used to establish growth curves. Two estimated feta l weight nomograms were produced from separate formulas: one combining biparietal diameter (BPD) and abdominal circumference (AC) and anothe r combining AC and femur length (FL). Percentile curves were determine d by regression analysis. Thirty-eight fetuses with an ultrasound betw een 15.0 and 22.4 weeks and cytogenetically confirmed trisomy 18 (n = 12) or trisomy 21 (n = 26) were evaluated. Results: For trisomy 18, th e BPD and AC formula sensitivities of the 5th, 10th and 50th percentil e cutoff values were 30, 60, and 70%, respectively. For trisomy 18, th e AC and FL formula sensitivities of the same percentiles were 40, 60, and 80%, respectively. For trisomy 21, the BPD and AC formula sensiti vities of the 5th, 10th, and 50th percentiles were 7.7, 11.5, and 69.2 %, respectively. For trisomy 21, the AC and FL formula sensitivities o f the same percentiles were 7.7, 19.2, and 73.1%, respectively. Conclu sions: For trisomy 18, the 10th percentile estimated fetal weight curv e had a sensitivity of 60% because of early fetal growth impairment. A pproximately 70% of trisomy 21 fetuses had an estimated fetal weight l ess than the 50th percentile; however, the lower sensitivities of the 5th and 10th percentiles limit the use of estimated fetal weight to de tect trisomy 21.