Passive smoking is linked to a significantly increased risk of miscarriage, stillbirth and ectopic pregnancy, finds a large observational study, published online in Tobacco Control.

The risk appears to be cumulative, with risk heightened in parallel with the length of time exposed to second hand smoke, the findings indicate.

It is well known that smoking during pregnancy significantly increases the risks of miscarriage and birth complications. What is less clear is whether passive smoking exerts similar effects, and if there are particularly critical periods of exposure to second hand smoke.

The researchers drew on historical data from a large sample of more than 80,000 women who had gone through the menopause, and been part of the Women's Health Initiative (WHI) study.

Just over 5000 of the women (6.3%) were current smokers; just under 35,000 (43%) were ex-smokers, who had smoked at least 100 cigarettes; and just under 41,000 (50.6%) were non-smokers. All had been pregnant at least once.

The group of non-smokers was then categorised according to the level of second-hand smoke they had been exposed to during childhood, as an adult at home, and as an adult at work.

The pregnancy outcomes showed that almost one in three of the entire sample (32.6%, 26,307) said they had miscarried at least once. Some 3552 (4.4%) had experienced a stillbirth, while 2033 (2.5%) had had a tubal ectopic pregnancy.

Not unexpectedly, younger and/or better educated women were less likely to miscarry or have birth complications; women of black and minority ethnic backgrounds and those who were overweight were more likely to do so.

Women who had never smoked were less likely to miscarry, have a stillborn child or an ectopic pregnancy than either current or former smokers, the data showed.

Compared with non-smokers, women who had ever smoked during their reproductive years were 16% more likely to miscarry, 44% more likely to have a stillborn child, and 43% more likely to have an ectopic pregnancy.

And these associations were also evident for non-smokers who had breathed in other people's cigarette smoke compared with non-smokers who had not been similarly exposed. The longer the period of exposure, the greater was the risk for non-smokers.

Those who had experienced the highest levels of lifetime exposure, including more than 10 years as a child, or more than 20 years as an adult at home, or more than 10 years in the workplace were 17% more likely to miscarry, 55% more likely to give birth to a stillborn child, and 61% more likely to have an ectopic pregnancy.