The 11-14 weeks scan - KH Nicolaides, NJ Sebire, RJM Snijders, AP Souka

Chapter 4

PREVALENCE AND EPIDEMIOLOGY

Twins account for about 1% of all pregnancies with two-thirds being dizygotic and one-third monozygotic. The prevalence of twins varies with ethnic group (up to 5-times higher in certain parts of Africa and half as high in parts of Asia), maternal age (2% at 35 years), parity (2% after four pregnancies) and method of conception (20% with ovulation induction). The prevalence of monozygotic twins is similar in all ethnic groups and does not vary with maternal age or parity, but may be 2-3 times higher following in-vitro fertilisation procedures, possibly because with these methods the architecture of the zona pellucida is altered although the mechanism remains uncertain.

In the last 20 years the rate of twinning has increased. This increase is most marked for dizygotic twins. It is estimated that about one third of the increase in multiple births is a consequence of childbearing later in life, but the majority of the effect is due to the use of assisted reproduction techniques.