3.2 Modeling fetal growth reference
As shown in Table S3, S4 and S5 , both the overall growth curve through 14 to 37 weeks of gestation and each fetal biometric measurement according to gestational age were compared among the three groups. The statistical differences occurred in the fetal biometric measurement curves for EFW, AC, FL and HL between SC-DCDA and SC-MCDA twins, and the size of each biometric measurement appeared to be smaller in SC-MCDA (allp <0.05). The growth velocity for EFW, BPD, HC, FL and HL of SC-DCDA twins was also different from those of SC-MCDA twins (allp <0.05), but no statistical differences were found in AC. When evaluating week-specific differences in the biometric variables between SC-MCDA and SC-DCDA twins, the results presented that EFW, FL and HL growth slowed down in SC-MCDA twins compared with SC-DCDA during most of the gestational weeks (p <0.05). In addition, the size of all the six fetal biometric measurements demonstrated smaller through 15, 21 and 25 weeks of gestation in SC-MCDA twins than that in SC-DCDA twins consistently (p <0.05). When the comparison between SC-DCDA and ART-DCDA groups was examined, significant differences were observed in all of the six fetal biometric measurements curves and only in the growth velocity for HC (all p <0.05). Similarly, week-specific differences of all the six fetal biometric measurements were significant at a few weeks of gestation (all p <0.05). Further focusing on SC-MCDA and ART-DCDA twins, it was found that there were significant differences in all of the six fetal biometric measurements curves and the growth velocity of EFW, BPD, HC, FL and HL. Meanwhile, week-specific differences of all the six fetal biometric measurements were statistically significant during most gestational weeks.
Tables S6, S7 and S8 showed that all of the SC-MCDA, SC-DCDA and ART-DCDA twins were different from singletons, especially during the third trimester.
Figure 2, 3 and 4 show the fitted growth curves of fetal biometric parameters, including EFW, BPD, HC, AC, FL and HL for SC-MCDA, SC-DCDA and ART-DCDA twins, respectively. The smoothed percentiles, such as the 5th, 10th, 50th, 90th, and 95th percentiles of fetal biometric parameters by gestational age are illustrated in Table S9 .