ABSTRACT
The rate of multiple pregnancies, mainly twins, has increased significantly in the past three decades, owing mostly to the introduction of assisted reproductive technologies in developed countries1. Multiple pregnancies are associated with higher risks of preterm birth, fetal anomalies and intrauterine growth restriction2,3. Twin pregnancies result from the fertilization of two ova (dizygotic twins, DZ) or a single ovum (monozygotic twins, MZ) (see Chapter 15). They can also be classified on the basis of chorionicity and amnionicity (see Chapter 24). All DZ twins are dichorionic-diamniotic, whereas MZ twins are dichorionic-diamniotic, monochorionic-diamniotic or monochorionicmonoamniotic, depending on the timing of zygote division4. Before the introduction of
assisted reproductive technologies (ART), the estimated proportions in twin pregnancies are 80% dichorionic and 20% monochorionic5.
