Screening for triploidy by the risk algorithms for trisomies 21, 18 and 13 at 11 weeks to 13 weeks and 6 days of gestation

Abstract

Objectives: To examine the effectiveness in screening for triploidy at 11 weeks to 13 weeks and 6 days of gestation by the combined use of the risk algorithms for trisomies 21, 18 and 13 based on maternal age, fetal nuchal translucency (NT) thickness, fetal heart rate (FHR) and maternal serum free beta-hCG and PAPP-A.

Methods: Prospective screening for trisomy 21, including 56 376 normal singleton pregnancies and 31 cases with triploidy (24 digynic and 7 diandric). Patient-specific risks for trisomies 21, 18 and 13 were calculated by multiplying the age related risk for trisomies 21, 18 and 13 with the likelihood ratio (LR) for fetal NT and with a combined LR for FHR, free beta-hCG and PAPP-A, respectively. Detection rates (DRs) and false positive rates (FPRs) for triploidy were calculated by taking the proportion with risks above a given risk threshold and standardized for maternal age according to the distribution of pregnancies in England and Wales in 2000 to 2002.

Results: In digynic triploidy, free beta-hCG was 0.16 (range 0.02-0.95) MoM and PAPP-A was 0.06 (range 0.02-0.47) MoM and the respective values for diandric triploidy were 8.74 (range 2.83-47.0) and 0.74 (range 0.32-1.51) MoM. Use of the trisomy 21 algorithm identified 5 of the 7 diandric cases and 3 of the 24 digynic ones at a 3% FPR. Combined use of the risk algorithms for trisomies 18 and 13 identified 20 of the 24 cases with the digynic triploidy at the FPR of 0.2%. When screen positivity was defined by a 3% FPR using the algorithm for trisomy 21 and an additional 0.2% FPR using the algorithms for trisomies 18 and 13, the total FPR was 3.1% and the DR was 84%.

Conclusions: A beneficial side-effect of first trimester combined screening for trisomies is the detection of a high proportion of fetuses with triploidy.