• mortality is 10 – 25%, the highest during the perinatal period
• growth of the twins depends on the following factors:
  • insertion of the umbilical cord (velamentous or marginal × central)
  • the existence and type of vascular anastomoses between the placentas
  • placental sharing between the twins (may be very unequal)

The fetus with a velamentous umbilical cord insertion and a distinctly smaller area of the placenta, from which it is fed, is in danger of chronic hypoxia, malnutrition and growth retardation. Discordant growth of the twins is a rule in twin transfusion syndrome.