ARE TWINS MORE LIKELY?

According to the Centers for Disease Control and Prevention in the USA, from 1980 to 2009 the twin birth rate doubled among white mothers alone in that country, and the increase was greatest among mothers over the age of 35, and particularly in mothers age 40 and older.

There are several possible reasons for this. First, women in their 30s experience greater fluctuations in hormone levels, especially follicle-stimulating hormones, which stimulate egg production for ovulation. Another factor may be related to weight – a study found that women who were overweight before conceiving were more likely to conceive twins than those who weren’t overweight. And a third factor is the increase in the use of fertility treatments among older mothers. “Epidemiological studies seem to indicate that women over 35 have a higher incidence of dizygotic pregnancies, where two separate eggs are fertilised by two separate sperm, although the incidence of monozygotic, or identical, twins isn’t increased in women over 35,” observes Johannesburg gynaecologist Kiran Kalian.

“By far the greatest contributor to the incidence of dizygotic twinning in women over 35 over the last few decades has been because of assisted reproduction such as IVF.”