A treatment to prevent premature births in single pregnancies may be ineffective in women expecting more than one child.
A study suggests an underlying difference between the way in which premature births occur in women with single and multiple pregnancies.
The findings, published in The Lancet, could help inform treatments to prevent premature births in women expecting more than one child.
The research could also help researchers understand how the process of premature labour may be different.
The study, led by Professor Jane Norman, looked at nearly 500 women around the country expecting twins.
Half of the women were given progesterone gel - a hormone which helps protect the lining of the womb. The other half were given a dummy version.
The treatment has been shown to reduce early births in women expecting one child considered at high risk of early labour.
However, researchers found that contrary to singleton pregnancies, the hormone gel did not reduce rates of premature births.
Multiple pregnancies accounted for 1.6 per cent of all births in the UK in 2007.
More than 98 per cent of multiple births are twins.
Around 20 per cent of multiple pregnancies result in pre-term delivery before 34 weeks compared to two to three per cent for women expecting one child.
Understanding the differences between what happens in premature births in singleton and multiple pregnancies can help us improve treatments and also address the higher incidence of premature labour in multiple pregnancies