Since Scotland introduced a ban on smoking in public places in 2006 there has been a 10% drop in the country's premature birth rate, say researchers.
They believe this is a smoke-free benefit that can be chalked up alongside others, like reductions in heart disease and childhood asthma.
Tobacco smoke has been linked to poor foetal growth and placenta problems.
Plos Medicine analysed smoking and birth rates for all expectant women in Scotland before and after the ban.
It included data for more than 700,000 women spanning a period of about 14 years.Significant change
Scotland was the first country in the UK to ban smoking in public places, followed by Wales, Northern Ireland and England in 2007.
After the legislation was introduced in Scotland, fewer mothers-to-be smoked - 19% compared with 25% before.
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It is important to remember that the reasons a baby can be born premature or underweight are complicated and that smoking is just one risk factor”
Andy Cole Chief Executive of Bliss At the same time there was a significant drop in the number of babies born prematurely or with low birthweight.
The investigators believe both are linked to the smoking ban, even though these rates started to go down some months before the ban was introduced and smoking incidence started to creep up again shortly after the ban.
They say there have not been any major changes in maternity care that would explain the findings.
Also, the reduction in premature births was both in non-smokers and women who continued to smoke when pregnant, which they say suggests passive smoke exposure is likely involved.But while their work suggests a link, it is not proof that one thing necessarily causes another. As with all retrospective studies like this, it is impossible to rule out entirely all other factors that might have influenced the finding.
However, Dr Daniel Mackay and colleagues from the University of Glasgow say their findings "add to the growing evidence of the wide-ranging health benefits of smoke-free legislation" and "lend support" to the adoption of such legislation in countries where it does not currently exist.
Andy Cole, chief executive of the special care baby charity Bliss, said: "We welcome the findings of this new study, which highlights a reduction in the number of babies born early or with low birthweight in Scotland, where around 8,000 babies are born each year needing specialist hospital care.
"Bliss always recommends that women should not smoke during pregnancy and that they should lead a healthy lifestyle. However, it is important to remember that the reasons a baby can be born premature or underweight are complicated and that smoking is just one risk factor."According to the British Heart Foundation, there are more than nine million smokers in the UK, and smoking remains the UK's biggest cause of avoidable early death.
It says the focus should now shift to the effect of smoking in the home and confined spaces, such as cars, especially where children are present.
A Scottish Government spokesman said: "We are continuing to build upon the achievements made to protect future generations from the devastating effects of smoking such as bans on cigarette vending machines and the displays in shops.
"We are committed to ensuring a new comprehensive robust tobacco control strategy for Scotland is developed this year. This strategy will focus on prevention and cessation and include ambitious targets for reducing smoking across Scotland."
