Determining the type of twins (zygosity)

Zygosity reflects the type of twin namely monozygotic or dizygotic.

Did the twins originate from 1 fertilized oocyte and split in 2 (= monozygotic) or did 2 oocytes got fertilized (= dizygotic)?

The correct zygosity diagnosis

Determination of the zygosity needs to be performed at birth using 4 criteria:

  • Sex
  • Structure of the placenta
  • Blood group
  • DNA

Female-male twins are always dizygotic. About 50% of the dizygotic twins have the opposite sex. During the examination of the placenta at birth, the number of chorionic membranes is counted. If only 1 chorionic membrane is present (= monochorial), the twins are certainly monozygotic. 75% of the monozygotic twins are monochorial. If the twins have the same sex, the same blood group, 2 chorionic membranes and 2 amniotic sacs, a DNA test needs to be performed to confirm the zygosity. If the genetic markers are different the twins are dizygotic, if the genetic markers are equal the twins are monozygotic.

The number of placentae doesn’t determine the type of twins. About 50% of the dizygotic twins have at first sight only 1 placenta at birth. 16% of the dizygotic twins have 2 placentae at birth. The number of placentae depends on the location of implantation of the embryo. If the embryos are implanted very closely, the placenta’s can fuse.

Why is it important to determine the zygosity?


  • Monozygotic twins are ideal partners for organ donation
  • In case of a genetic disease


  • Every parent and twins have the right to know the precise zygosity.
  • The zygosity has important implications for the twins and their family, friends, school and work. Every individual needs to identify themselves and distinguish themselves from others.
  • It is important to recognise every twin as an individual. If the similarities or differences between the twins are emphasised, the development of the child can be negatively affected.
  • Monozygotic twins deserve special attention during their development. The children need to discover that they are each a unique individual although they have so many similarities.


  • In scientific research dizygotic and monozygotic are often compared to distinguish genetic factors from environmental influences. Monozygotic twins are mostly more alike than dizygotic twins.