Pregnancy & Baby 

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Twins: What are my chances of having a second set?

There are two types of twinning – monozygotic (identical) and dizygotic (fraternal) twins. The frequency of identical twins across the world is approximately one set per 250 births and is generally not dependent on race, heredity, age or parity (number of pregnancies). Ovulation induction doubles the incidence of this type of twinning. Fraternal twin conception is influenced by race, heredity, maternal age, parity and, especially, fertility drugs. Regarding heredity, the mother is most responsible for this. In one study, women who themselves were fraternal twins gave birth to twins at a rate of one set per 126 pregnancies. In another study, one mother of twins out of every 25 (four per cent) was also a twin, but only one out of 60 (just under two per cent) of their fathers was a twin.

As maternal age and parity increase, so does the incidence of fraternal twinning. There is a tendency for fraternal twins to repeat in families. If the firstborn are twins of this type, you are about five times more likely to repeat twinning with the next pregnancy than would be true in the general population.

A good resource for all things twin is the Multiple Births Foundation based at the Institute of Obstetrics & Gynaecology, at Queen Charlotte's and Chelsea Hospital in London. The Foundation was established to support professionals working with multiple birth families, and runs specialist clinics for families. Their website is at www.multiplebirths.org.uk

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