Determination of chorionicity in twin placentas is done by studying the intertwin septum. Twins are either monozygotic (30% of all twin pregnancies) or dizygotic (70%). Dizygotic twins result from the fertilization of two different eggs and have per definition a dichorionic (DC) placenta, i.e. the septum consists of two layers of amnion with a double central, usually fused layer of chorion (figure 1). Chorionicity of monozygotic twins depends on timing of splitting of the fertilized egg. If splitting occurs within three days after fertilization, the monozygotic twin will also have a DC placenta. Splitting after the third day results in monochorionic (MC) placentation, i.e. the septum consists of two layers of amnion without chorion (figure 1). Of these monozygotic twins two-thirds are MC, see also figure2.