Background Few studies have reported on ventriculomegaly (VM) in twins or in relation to chorionicity.

Methods All twin pregnancies with suspected VM (n=67) occurring in Northern England during September 1994–December 2008 formed this population-based study. Data were obtained by cross-referencing with the Northern Congenital Abnormality Survey, the regional fetal medicine unit database, and the Northern Survey for Twins and Multiple Pregnancies.

Results VM was confirmed in 63 twin fetuses (26 monochorionic (MC); 37 dichorionic, DC). Prevalence rates during 1998–2008 were 56/10 000 twin births (95%CI 52 to 73), 122/10 000 MC births (95%CI 82 to 181) and 46/10,000 DC births (95%CI 32 to 64). Concordance for VM in MC and DC twins was 23.1% and 10.8% respectively.

MC twins: Eight cases occurred in pregnancies complicated by twin-twin-transfusion syndrome. Ten had prenatally detected associated anomalies (80% were nervous system anomalies); 4/10 were selectively terminated (sTOP). 2/26 had growth-restriction and 1/26 co-twin demise. Five were apparently isolated prenatally but three had anomalies diagnosed after delivery.

DC twins: 16 cases had additional anomalies; eleven requested sTOP (nine in the third trimester; 2/9 laboured before the procedure could be performed). 3/37 followed co-twin demise, incorporating two livebirths with resolved VM. Seventeen cases were apparently isolated; VM persisted in all. The median gestation and atrial size at presentation were 25 weeks (IQR: 22–21) and 14.0 mm (IQR:11.0–21.5) respectively. 2/17 were sTOP (cephalocentesis required in one) and 15/17 were liveborn. Additional structural anomalies were identified following post-mortem or postnatal examination in five cases.

Conclusion Valuable outcome data relating to VM in twins is provided which will help with pre-natal counselling.