Prenatal screening of trisomy 21 in Peru through the first-trimester contingent combined plus screening test

Introduction. The combined first-trimester screening test is the international recommendation for prenatal screening of trisomy 21. The combined plus screening test adds one or more ultrasound markers (ductus venosus, tricuspid flow, nasal bone). The contingent test is a two-step protocol of risk assessment that considers all markers only as a contingency in the group with an inconclusive result. Objective: Assessment of the performance of the Kagan biochemical contingent protocol in our population. Methods. All pregnant women between 11 and 13 weeks attended in our unit were assessed with the first-trimester combined plus screening test. We followed the Fetal Medicine Foundation published quality criteria and the International Society of Ultrasound in Obstetrics and Gynaecology guidelines. Results 2 578 single pregnancies were assessed; the foetuses were classified by an initial ultrasound evaluation; 37 (1,4%) presented high risk (>= 1/50), 1 888 low risk (< 1/1 000) and 653 intermediate risk (between 1/51 and 1/1 000). Contingent PAPP-a and free beta-hCG were recommended to pregnant women with intermediate risk. 46 (1,8%) other foetuses were classified at high risk for trisomy 21 following biochemical testing. All trisomy 21 foetuses in our cohort were among them. Pregnant women over the age of 36 (at ultrasound date) or 38 (at term) were considered with basal high risk (BHR basal risk >= 1/200), since all of them needed biochemical testing. Conclusions: Our results validated Kagan's protocol. The false positive rate and the size of the intermediate risk subgroup would depend on the maternal age distribution.