Aneuploidy screening and diagnostic tests
All pregnant women were provided with information about aneuploidy screening or diagnostic tests. Basic information included the limitations, test performance and difference between screening and diagnostic test. The screening tests included MSS and cfDNA screening. MSS included combined dual, triple, quad, integrated and sequential tests. cfDNA screening was analyzed for trisomies 21, 18, 13. The diagnostic tests included chorionic villus sampling and amniocentesis for conventional karyotyping. Prental chromosomal microarray has not been used clinically in Korea during this study period. The cfDNA screening or diagnostic test was recommended when NT was more than 3mm or 95th percentile, and the diagnostic test was recommended when fetal structural defects were observed on ultrasound according to the ACOG guideline [2]. After counseling, participants selected the first tier test. Based on the first-tier test, we divided all the participants into three groups: MSS, cfDNA screening, and IT. When the screening test showed a high risk result, the participants were counseled about further evaluation. In addition, participants were offered detailed, high-resolution ultrasound examinations in the second trimester.