Patients
From January 2017 to January 2018, five women carrying DCDA twins with one aneuploidy fetus were enrolled in this study (Table 1). The median maternal age was 38.8 years and 80% (4/5) of pregnant women had a maternal age above 35 years. All women were conceived by in-vitro fertilization and embryo transfer (IVF-ET), and each woman had received the transplant of two embryos. During routine prenatal healthcare, all women had at least one risk factor of fetal aneuploidy, including advanced maternal age, increased fetal nuchal translucency (NT) thickness, and structural abnormalities in ultrasound examination (Table 1). Fetal aneuploidies were confirmed by CVS or amniocentesis, showing four women with a co-twin of trisomy 18 (T18) and one woman with a co-twin of trisomy 21 (T21). The median of gestational age receiving reduction procedure was 15 weeks. After reduction, each woman voluntarily donated her blood samples every two to five weeks until delivery. Depending on their willingness, patients 1, 2, and 5 gave blood for seven times, five times, and six times after reduction, respectively. Patient 3 and 4 both gave blood for nine times after reduction.