3. Results
A total of 4055 pregnancy women, 2142 singleton vs. 1913 twin, were recruited in this study. The flowchart of twin participants enrollment was shown in Figure 1 (The enrollment flowchart for singleton participants is very similar and not shown). The final sample consisted of 2019 singleton, alongside with a total of 9787 ultrasound observations, and 929 mothers (148 SC-MCDA, 215 SC-DCDA and 566 ART-DCDA), alongside with a total of 12837 ultrasound observations (3099 in 296 SC-MCDA, 2542 in 430 SC-DCDA and 7196 in 1132 ART-DCDA fetuses). A gestational age interval between 14 and 37 weeks was covered.