Twins achieved through in-vitro fertilisation often undergo a viability ultrasound at 6-9 weeks of gestation. The presence of inter-twin crown-rump length discordance at this stage is not an uncommon finding; however the clinical significance of this is unknown. We analyzed 218 dichorionic twin pregnancies, producing two live fetuses > 24 weeks gestation, to determine whether inter-twin discordance (> or = 85th centile) in the mid-first trimester was associated with birthweight discordance (> 20 ), or small for gestational age (<10th centile). The incidence of birthweight discordance and small for gestational age infants were determined, with no increased risk found for the discordant population. This may provide some reassurance to treating clinicians.