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This study aimed to compare the associations of crown–rump length (CRL) discrepancy with birthweight discordance in spontaneous versus vitro fertilization (IVF) conceived dichorionic twin pregnancies.A computerized retrospective study of women referred to our ultrasonographic unit for nuchal translucency examination between January 1997 and December 2011. The study group was subdivided into twins conceived after IVF, non-IVF fertility treatment and spontaneously conceived twins. Birthweight discordance was defined as a difference of birthweights of >20%.A total of 688 dichorionic twin pregnancies were included, all of them ending in live birth of both twins. IVF-conceived pregnancies were associated with a significant increased risk of extreme birthweight discordance compared with spontaneous-conceived twin pregnancies (OR = 2.3; CI = 1.4–3.8, P < 0.001). A significant correlation was found between CRL discrepancy and birthweight discordance in spontaneous-conceived pregnancies (r = 0.15, P < 0.05). In contrast, there was no significant correlation between CRL discrepancy and birthweight discordance in both IVF and fertility treatment-conceived pregnancies.Because birthweight discordance in IVF-conceived twins did not correlate with CRL discrepancy, we assume that it emerges later in pregnancy, maybe related to maternal underplaying complications, for which IVF was indicated in the first place. © 2014 John Wiley & Sons, Ltd.What's already known about this topic?There is conflicting evidence regarding the association between CRL discrepancy with birthweight discordance.What does this study add?This study found a significant correlation between CRL discrepancy and birthweight discordance in spontaneous-conceived dichorionic twin pregnancies. In contrast, there was no significant correlation between CRL discrepancy and birthweight discordance in both IVF and fertility treatment-conceived pregnancies.