During a four-year period 30 patients were diagnosed by sonar in the first trimester of pregnancy as having twin conceptions. Of these 30 patients, 14 eventually gave birth to twins. Eleven of the remaining 16 patients were found to have a normal pregnancy and a coexistent blighted ovum (empty sac), and ultimately all but one gave birth to a single baby; the eleventh patient aborted a fresh fetus at 25 weeks of gestation. Of the last five patients, four were diagnosed as having twin blighted ova, and one a blighted ovum and missed abortion; these five patients either aborted spontaneously or had an elective termination of pregnancy. The results suggest that the incidence of twin conceptions is higher than commonly accepted figure of 1 in 80.