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Capillary hemoglobin values estimated simultaneously in liveborn partners of 86 pairs of dichorionic twins during the first week of life were comparable in the two partners. In 20 pairs of liveborn twins with monochorionic placentas the values were different, with the second-born having the higher hemoglobin concentration. This difference, which was small in the first hours after delivery but became greater later (12 to 24 hours), indicates that shunting of blood occurs during delivery. Because of the abundant and easily recognizable anastomoses this placentofetal transfusion is greater in monochorionic twins. Ergometrine injection immediately after delivery of the second twin may enhance this process. Estimations of venous hemoglobin in 17 dichorionic twins showed slightly higher values in the second-born partners. The corresponding capillary blood levels were an average of 2.5 Gm./100 ml. higher than the venous values.

Twins may be dizygotic or monozygotic. Of the monozygotic twins, 25% to 30% are dichorionic and 70% to 75% are monochorionic. Whilst all the dichorionic twins are diamniotic, very occasionally some monochlorionic ones may be monoamniotic; some of these may be conjoined. A case of conjoined twins was diagnosed during pregnancy. The two girls were delivered by elective classical caesarean section. They were joined together from just above the xiphisternum to the umbilicus and were successfully separated. Recent literature is reviewed.