The chances of a miscarriage rise sharply if a woman's partner smokes heavily during her pregnancy, research has found.
Smoking has a number of effects on health
A study found that nearly a third of women whose partners smoked more than 20 cigarettes a day lost their babies within six weeks of conceiving.
Among those who partners did not smoke, the rate was a fifth.
The research, by researchers in the US and China, is published in the American Journal of Epidemiology.
Previous research into the effect of passive smoking on pregnancy has produced inconclusive results.
This latest study focused on 526 Chinese textile workers who were newly-married.
It found that whether or not a woman's partner smoked had little difference on the likelihood of conception.
But it did seem to have a significant effect on whether the pregnancy continued past the early weeks if the partner smoked 20 or more cigarettes a day.
In all, 84% of the women whose husbands did not smoke eventually fell pregnant
and gave birth.
Among those whose partners did smoke the rate was 76%.
The study was conducted by the Harvard School of Public Health in America and
Beijing Medical University in China between 1996 and 1998.
Researcher Dr Scott Venners said the study suggested that some of the babies lost by women whose partners smoked would otherwise have survived to full term.
The scientists believe that smoking could cause damage to the chromosomes in
In addition, exposure to tobacco smoke breathed in by a pregnant woman may endanger the developing foetus by affecting levels of female sex hormones, or by reducing blood flow through the placenta.
Professor Alison Murdoch, president of the British Fertility Society, said: "Everyone in a household where there is going to be a baby should stop smoking."
However, Simon Clark, of the smokers' rights organisation Forest, told BBC News Online it would be wrong to jump to conclusions as the associations uncovered in the study were weak.
He said: "More research is needed because this report doesn't prove anything."