Passive smoking leads to earlier delivery of babies
Women who are exposed to passive smoking while pregnant deliver their babies earlier and with lower birth weights, research has found.
The study, by the University of Bristol, involved more than 5,000 women delivering their babies in Bristol between 2012 and 2014.
Researchers used both a woman's report of her exposure to passive smoking, as well as an exhaled carbon monoxide reading in early pregnancy.
Many other factors, including maternal age, weight and ethnicity, employment and social circumstances, were also considered.
The research team found that non-smoking women that lived with a smoker were more likely to deliver their babies earlier compared to those who did not live with someone who smoked.
Women exposed to passive smoking also had smaller babies, the study, published online in Reproductive Sciences, found.
The effect on gestational age at delivery was around a third smaller than when a woman herself smoked, however more women are affected by passive smoking.
About one in seven women smoke through their pregnancies but more than one in four non-smoking women live with a smoker.
Dr Rachel Ion, from Bristol's School of Clinical Sciences and St Michael's Hospital, said: "Women are generally aware of the risks of smoking during pregnancy but more education is needed to inform women and their families and friends of the emerging evidence of the risks associated with passive smoking in pregnancy.
"Our results add evidence to public health arguments to implement further measures to reduce exposure to passive smoking."
Babies that are born too early can suffer from complications, with premature births accounting for more than a million deaths each year worldwide.
Those babies that survive can face significant long-term problems including breathing difficulties and cerebral palsy.
Premature birth also affects the child's family and society as a whole, with significant healthcare costs associated with caring for them.
The study did not examine these long-term outcomes but the finding that passive smoking is associated with earlier delivery highlights the importance of studying such environmental factors, the researchers said.