OBJECTIVE To assess whether early fetal growth restriction in a twin, expressed as the intertwin discrepancy in crown-rump length (CRL) between two viable twins at 7 + 0 to 9 + 6 weeks' gestation, is predictive of subsequent single fetal loss. METHODS This was a retrospective analysis of data collected prospectively over 3 years. Women attending an early pregnancy unit underwent a transvaginal ultrasound examination. In women with a viable twin pregnancy between 7 + 0 and 9 + 6 weeks of gestation, the CRL ratio between the two embryos was calculated. Women were followed up and the intertwin discrepancy in CRL at the 7 + 0 to 9 + 6-week scan was compared between cases in which there was spontaneous reduction to a singleton and those in which both twins remained viable at the 11-14-week scan. RESULTS There were 77 women included in the study and nine (12%) of these had a single fetal loss. Pregnancies with subsequent single fetal loss were more likely to have a larger median CRL discrepancy (42.0%; interquartile range (IQR), 23.8-64.3%) than were those which retained two viable fetuses (6.1%; IQR, 2.2-12.5%) (P < 0.0001). The median CRL discrepancy in subsequently viable monochorionic diamniotic twins (10.9%; IQR, 1.9-17.5%) was no different from that in dichorionic diamniotic twin pregnancies (5.9%; IQR, 2.1-12.3%) (P = 0.305). Regardless of chorionicity, there was a relationship between increasing CRL discrepancy and single fetal loss; the likelihood of a subsequently viable twin pregnancy was 97% if the discrepancy was < 20%, while if the discrepancy was > 60% there were no cases of both twins remaining viable (P < 0.0001). Receiver-operating characteristics curve analysis of CRL discrepancy in predicting single fetal loss gave an area under the curve of 0.93, with an optimum cut-off point of 16.9% discrepancy (sensitivity, 88.9%; 95% CI, 51.8-99.7; and specificity, 86.7%; 95% CI, 76.3-93.8). CONCLUSIONS There is a significant relationship between CRL discrepancy at 7 + 0 to 9 + 6 weeks and the likelihood of subsequent single intrauterine fetal loss. This suggests that spontaneous fetal demise of one twin may be preceded by growth restriction in the first trimester.