Ultrasound determination of chorionicity is most reliable before 14 weeks and subsequently reduces in accuracy with advancing gestation. The aim of this study was to evaluate the accuracy of chorionicity determination in a birth cohort of twin pregnancies. Antenatal ultrasound determination of chorionicity in twin pregnancy was undertaken at suboptimal period of gestation in over twothirds of twin pregnancies. Even in the first trimester of pregnancy, the accuracy of chorionicity determination was lower than previously reported. DOI: http://dx.doi.org/10.4038/cmj.v58i4.6309 Ceylon Medical Journal 2013; 58: 170-172