LONDON (Reuters) - Giving women pregnant with twins the hormone progesterone does not appear to prevent premature birth despite showing promise in doing so with single pregnancies, British researchers said on Thursday.

Multiple pregnancies carry a bigger risk for women during pregnancy and significantly increase the likelihood of miscarriage, premature birth and long-term health problems for the child. They can also endanger a mother's health.

Previous studies had shown progesterone treatment might prevent premature birth in some high-risk pregnancies but the findings published in the journal Lancet suggest the same does not hold true for twins.

"The clinical implication of our study is that progesterones are not effective in women with twin pregnancy for prevention of preterm delivery," Jane Norman of the University of Edinburgh and colleagues wrote.

In the so-called STOPPIT study of 500 women who were pregnant with twins between 2004 and 2008, the researchers started giving the expectant mothers either daily progesterone treatment or placebo for 10 weeks.

The team also reviewed published and unpublished data on the hormone's effectiveness at preventing pre-term delivery in twin pregnancies, finding the treatment did not reduce the likelihood of premature delivery or a foetus dying in utero.

The proportion of women who delivered or who had a baby die before 34 weeks of pregnancy was 25 percent in the progesterone group and 20 percent among those who received the placebo.