Just being near their partners' smouldering cigarettes damages non-smoking women's chances of pregnancy through IVF, researchers say today.

The effects are as damaging as if women are smokers themselves, according to a Canadian study.

Non-smokers achieved pregnancy rates about two and a half times better than either smokers or so-called side-stream smokers. These are people who are exposed to toxins in smoke from cigarettes, rather than, as in passive smoking, both lit cigarettes and smoke exhaled by the smoker.

The impact on IVF treatment from smoking has been known for some time, but the study in the journal Human Reproduction raises the need for more research into how smoke damages women's fertility.

Researchers from McMaster University and Hamilton Health Sciences in Hamilton, Ontario examined the quality of embryos, implantation and pregnancy rates of 225 women undergoing IVF or ICSI, where an egg is fertilised by injecting a single sperm.

They were grouped into non-smokers, smokers and side-stream smokers living with a partner who often smoked.

Michael Neal, a PhD candidate leading the research, said the pregnancy rates were 48% for non-smokers, about 19% for smokers and 20% for side-stream smokers.

Implantation rates were 25% for non-smokers, but only 12% for smokers and side-stream smokers.

Warren Foster, director of IVF and reproductive biology at the centre for reproductive care at McMaster University, said: "Our study warrants a warning to women to reduce or prevent exposure to cigarette smoking, especially if they are trying to conceive."