New reference values for biometrical measurements and sonographic estimated fetal weight in twin gestations and...

Objective To construct new reference values for biometrical measurements and sonographic estimated fetal weight (sEFW) in twin gestations and compare them to previously published normograms. Methods A retrospective analysis of sEFW evaluations of twin gestations was performed between 2011 and 2016 in a single university-affiliated medical center. sEFW was calculated using the Hadlock 1985 formula. To avoid selection bias, one evaluation per pregnancy was randomly selected. Following mathematical transformation to obtain normality of values, normograms were constructed using a best-fit regression model for estimation of mean and standard deviation at each gestational age (GA). Normograms were validated by applying all observations to ensure equal distribution at parallel percentiles. Our normograms were then compared to previously published sEFW normograms for twin gestations. Results A total of 864 sEFW evaluations were performed on 195 twin pregnancies at 22-39 gestational weeks. Of them, 390 entered the primary analysis. The rest were left for validation. Seventy percent of the cohort were dichorionic-diamniotic twins (136/195), 16% (32/195) were monochorionic-diamniotic twins and three (1.5%) were monochorionic-monoamniotic twins. Twenty-four fetuses lacked data on chorionicity. The rest were monochorionic twins or were of unknown chorionicity. Values corresponding to the 2.5th, 10th, 50th, 90th and 97.5th percentiles for sEFW are presented for every GA. Validation by applying all 864 evaluations on constructed normograms was achieved. Comparison to previously published twins' sEFW normograms demonstrated wide variation between curves. Conclusion New reference values for biometrical measurements and sEFW in twin gestations are presented for clinical and research use. Comparison to other curves demonstrates the wide variability and need for further investigation on twin's normal growth.