4.3 Clinical implications
The accuracy of intrauterine growth assessment for twins depends on the establishment of twin-specific growth charts, and the longitudinal ultrasonographic standards have benefits in recognizing growth pattern variations at different gestational ages31. The international society of obstetrics and gynecology ultrasound emphasized the clinical significance of development and use of twin-specific growth charts when assessing fetal growth of twins since 201632. In medical practice, it has been well-known that growth restriction in twins is prevalent due to the slower growth rate in the third trimester5-8. In our study, all the median (50th percentile) biometric parameters of SC-MCDA, SC-DCDA, ART-DCDA twins were lower at each gestational week compared with fetal biometry reference of Chinese singleton33, 34. Therefore, over-diagnosis of restricted intrauterine growth should be a common concern when adopting the diagnostic criteria based upon the standard of singletons. For a long time, although there are the continuous efforts, trying to establish fetal growth curve for twins, but the issue has not been resolved. The existing data has inherent limitations, for example, some of the data came from birth weight and cannot be applied to the growth and development assessment during pregnancy24, 35-38, some others recruited subjects by small sample size5, 11, 15 or without exclusion of high-risk pregnancy5, 12, 16. In the present study, we got over the limitations and largely promised our growth reference chart customized for chorionicity and mode of conception a more reliable tool to distinguish cases with fetal growth restriction in twins39.
We are aware that downgrading the fetal growth reference may sacrifice the sensitivity to identify pathological fetal growth restriction. Our subject enrollment strategy adopted strict inclusion criteria for pregnant woman, which ensured a qualified sensitivity in the early screening of fetal growth restriction. Notably, our sampled population included pregnancies via ART. Since twin pregnancies via ART accounts for more than half of twins, our growth charts customized for conception mode would be more generalizable to the current population of twins in China. The results of this study illustrated an asymmetric pattern of growth velocity between DCDA and MCDA twins, which has been confirmed by previous studies 6, 11, 16. In addition, it has been suggested that chorionicity has a significant independent effect on birthweight40, and the threshold of physiological intertwin size discordance of fetal biometry also vary according to the chorionicity41. In this case, developing growth reference charts customized for chorionicity is necessary.
In this study, six fetal biometric measurements obtained from ultrasound were opted to customize the growth reference, but not only the EFW. A previous study found that some biometric measurements may vary according to parental ethnicity or different constitutional characteristics, not all the differences can be specifically explained by the changes of EFW6. As proposed by previous studies42, 43, we developed all fetal biometric parameters growth charts rather than only EFW in most studies8, 12, 13. The full-spectrum parameters could enrich our knowledge on fetal changes in uterus, which should be clinically significant to promote a more comprehensive evaluation of fetal intrauterine growth.