Is ultrasound alone enough for prenatal screening of trisomy 18? A single centre experience in 69 cases over 10 years

Abstract

OBJECTIVES: To evaluate ultrasound scan and other prenatal screening tests for trisomy 18 in a regional obstetric unit and to review the management approach for women with positive trisomy 18 screening results. METHODS: Prenatal diagnosis databases were accessed to identify fetuses that had confirmed trisomy 18 karyotypes or were at high risk for trisomy 18 on second-trimester biochemical screening or first-trimester combined screening tests over a period of 10 years from 1 September 1997 to 30 September 2007. RESULTS: Sixty-nine women were confirmed to have trisomy 18 fetuses by karyotyping either prenatally (n = 61) or postnatally/post-miscarriage (n = 8) during the study period. The detection rate of ultrasound scan ≤ 14 weeks and 18 to 21 weeks to detect trisomy 18 was 92.7 and 100%, respectively. A total of 80 and 87% of fetuses had two or more ultrasound abnormalities detected in the ≤ 14 weeks and 18 to 21 weeks anomaly scans, respectively. Forty-eight women screened positive for trisomy 18 by second-trimester biochemical screening with human chorionic gonadotrophin (hCG) and alpha fetoprotein (AFP). Only one was true positive (positive predictive value = 1/48 or 2%). Eleven women screened positive for trisomy 18 by first-trimester combined screening with nuchal translucency scan and maternal serum for pregnancy-associated plasma protein A (PAPP-A) and hCG between 11 and 13 + 6 weeks. Three were true positive (positive predictive value = 3/11 or 27%). All four cases with positive screening had ultrasound abnormalities. CONCLUSIONS: Ultrasound scan for fetal anomalies is the most effective screening test for trisomy 18. A policy of conservative management for women with positive second-trimester biochemical screening or first-trimester combined screening for trisomy 18 is reasonable in the absence of ultrasound fetal abnormalities. Unnecessary invasive tests can be avoided.