Last year the total number of twin births, 123,536, was the lowest since 2002.

In 2007, at the height of the roaring twinties, twin births occurred at twice the rate of 1980.

A sizeable reduction in the rate of twin births would thus contribute significantly to the 100 million or so that it would cost to provide three full cycles.

Twin births are very rare - only 0.7% of the births observed in one study.

Newborn turs weigh 3.4to4.1 kg; they are usually singletons, although about 3% of pregnancies result in twin births.