Background Antenatal ultrasound studies report similar transcerebellar diameter (TCD) measurements within monochorionic twin pairs regardless of growth concordance. Biparietal diameter (BPD) studies demonstrate less consistent results. The cerebellum is recognised as having an important role in later behaviour and cognition. Recent research shows reduced cerebellar growth in IUGR singletons. We hypothesise that monochorionic twins treated for twin-twin transfusion syndrome (TTTS) will show differences in cerebellar measurements as well as BPD.

Methods The study population comprised twenty three MCDA twin pregnancies with TTTS treated by fetoscopic laser photocoagulation between 16 and 24 weeks gestational age (mean 20 weeks). Fetal MRI scan was performed approximately 5 weeks post treatment (range 2–10 weeks). BPD and TCD were measured for each twin and analysed using paired samples t-test. Twins with signs of congenital or acquired brain injury were excluded.

Results BPDs as well as TCDs and cerebellar vermis height were significantly different (p<0.01) within twin pairs affected by TTTS. However the ratio of TCD to BPD was significantly greater in the smaller twin (p<0.01).

Conclusion We report the novel finding of significant intrapair differences in both BPD and cerebellar parameters measured by fetal MRI in twins treated for TTTS. There appears to be some sparing of the cerebellum relative to cerebral hemisphere growth, possibly due to its anastamotic blood supply and reflecting its importance as an evolutionary structure. Cerebellar underdevelopment may underlie later impairments seen in smaller twins.