The loss of a twin during early pregnancy explains the increased risk of birth defects in multiple IVF pregnancies. This “vanishing twin” phenomenon is linked to a nearly two-fold increased risk of congenital malformation in the surviving baby, and a threefold increase in multiple birth defects.
The phenomenon occurs when there are fewer babies born than detected in early pregnancy by ultrasound - often reducing from twins to single babies but also possibly from triplets to twins.
Read more at University of Adelaide