Study population and study design
This was a prospective observational study in women of DCDA twin pregnancy opted for selective reduction of a co-twin in the Department of Fetal Medicine and Prenatal Diagnosis, the Third Affiliated Hospital of Guangzhou Medical University, China. Each woman has been previously confirmed to carry dizygotic twin pregnancy by ultrasound, and opted for fetal reduction due to an aneuploidy co-twin diagnosed by chorionic villus sampling (CVS) or amniocentesis. Before reduction, each woman was given a detailed explanation of the study, and written informed consent of participating in this study was obtained. All women were above 18 years old, and their gestational ages were eight weeks or above at the time of joining the study. Chromosomal abnormalities have not been found in women and her husband, neither the couples have received a blood transfusion, organ transplantation, cell therapy, or immunotherapy before pregnancy. Fetal reductions were conducted by transabdominal intracardiac injection of potassium chloride as the standard of practice [13]. Immediately before and twice a month after reduction till birth, the maternal blood of each woman was collected for NIPT for fetal T21, T18, T13 to evaluate the residual cfDNA effect of the reduced fetus. The couple’s age, gestational age at each time of blood sampling, ultrasound scanning results, prenatal diagnosis results, and other relevant clinical information were recorded. This study was approved by the Ethics Committee of the Third Affiliated Hospital of Guangzhou Medical University (no.2019096) and the Institute Review Board of BGI-Shenzhen (BGI-IRB 19081).