Combined first trimester screening for trisomy 21: Assessment of excess risk in case of free ß-human chorionic gonadotrophin between 5 and 10 multiples of the median.

Objective: The first trimester combined risk of trisomy 21 is obtained by multiplying the risk related to maternal age by the likelihood ratios of nuchal translucency, free beta-human chorionic gonadotrophin (β-hCG) and placenta associated plasma protein-A. Beyond five multiples of the median (MoM) of β-hCG, the risk of trisomy 21 is truncated. The objective of the present study was to evaluate the evolution of the first trimester combined risk of trisomy 21 in individuals with first-trimester free-β-hCG levels between 5 and 10 MoM.

Methods: We conducted a non-interventional cohort study from a 6-year database of combined first-trimester trisomy 21 screening of all individuals who underwent the screening in a French specialized medical analysis center. We included all pregnant individuals who had a serum-free β-hCG between 5 and 10 MoM. Patients for whom the status of the fetus, with or without trisomy 21, was not identified by the outcome of the pregnancy or by a karyotype result were excluded from the study. The discriminatory capacity of free-β-hCG above 5 MoM was studied by a receiver operating characteristic curve. We used an orthogonal polynomial regression to represent the evolution of likelihood ratios according to free-β-hCG in MoM.

Results: Among 413 216 combined first-trimester screens of trisomy 21, 2239 (0.5%) screens met the inclusion criteria. In the selected population, 801 (35.8%) were excluded from the study because of missing fetal or neonatal status, and 46 (3.2%) fetuses out of 1438 included were diagnosed with trisomy 21. For free β-hCG values between 5 and 10 MoM, the area under the curve is 0.56 (0.46-0.65). The scatterplot of the likelihood ratio of β-hCG showed an increasing parabolic pattern: the likelihood of trisomy 21 increases with the free-β-hCG threshold.

Conclusion: To override the truncated risk of trisomy 21 in case of free β-hCG values between 5 and 10 MoM, the study has allowed us to estimate the adjusted risk of trisomy 21, enabling health professionals to offer appropriate prenatal counseling.