Women exposed to passive smoke have a significantly increased risk of suffering a miscarriage, stillbirth or ectopic pregnancy, a new study has found.

According to US scientists, this risk appears to be cumulative - in other words, the longer a woman is exposed to secondhand smoke, the greater the risk.

Those exposed over the longest times were found to be 17% more likely to suffer a miscarriage and 55% more likely to suffer a stillbirth.

It is already known that smoking during pregnancy increases the risk of miscarriage and complications related to the birth. However the risks associated with passive smoking are less clear, so the scientists decided to investigate this further.

They analysed data relating to over 80,000 women who had already been through the menopause. Just over 5,000 (6%) of these were current smokers, while almost 35,000 (43%) were ex-smokers. The remaining women were non-smokers.

All of the women had been pregnant at least once in their lives.

The scientists divided the non-smokers into categories based on how much passive smoke they had been exposed to as children, as adults in their home and as adults at work.

Overall, almost one in three of the women had suffered a miscarriage, while just over 4% had suffered a stillbirth and 2.5% had experienced an ectopic pregnancy.

The study found, as expected, that younger and/or better educated women were less likely to miscarry or suffer complications during the birth. Overweight women and those from ethnic minority backgrounds were more likely to miscarry or experience birth complications.

Women who had never smoked were less likely to suffer a miscarriage, a stillbirth or an ectopic pregnancy compared to current or ex-smokers.

The scientists found that women who had ever smoked during their reproductive years were 16% more likely to suffer a miscarriage than non-smokers. They were also 44% more likely to suffer a stillbirth and 43% more likely to have an ectopic pregnancy.

However, the scientists also found that these risks existed for non-smokers who had been exposed to passive smoking, compared to non-smokers who had not. The longer they were exposed to secondhand smoke, the bigger the risk.

The study found that women who had been exposed to the highest amounts of passive smoke during their lifetime - this included 10 years as a child, more than 20 years as an adult at home and more than 10 years as an adult at work - were 17% more likely to suffer a miscarriage.

They were also 55% more likely to suffer a stillbirth and 61% more likely to have an ectopic pregnancy.

Details of these findings are published in the journal, Tobacco Control.

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