Women undergoing IVF should avoid other people's smoke to have the best chance of a successful pregnancy, say experts.
Side-stream smoking comes from smoke emitted from a lit cigarette
Smoke from other people's cigarettes more than halved the likelihood that a transferred embryo would take and grow, a team of Canadian researchers found.
It is already known that smoking can hamper a woman's fertility, but this latest study hints that exposure to others' smoke might be just as bad.
The McMaster University research is published in Human Reproduction.
Senior researcher Professor Warren Foster said: "Although we do need a prospective confirmation study, the findings from our study already warrant a warning to women to reduce or, if possible, prevent exposure to cigarette smoking, especially if they are trying to conceive."
His team, along with colleagues at Hamilton Health Sciences in Ontario, studied 225 women undergoing IVF or another fertility treatment called ICSI.
They divided the women up into groups according to their smoking status - smokers, non-smokers or "side-stream" smokers, which meant that the women were living with a partner who regularly smoked and therefore exposed to smoke emitted from lit cigarettes.
They found no difference in the quality of the embryos from the three groups.
But there was a striking difference in implantation and pregnancy rates between the non-smoking group and the smokers and side-stream smokers.
Lead author Dr Michael Neal said: "There was a significant difference in the pregnancy rates per embryo transfer, with the non-smokers achieving around 48%, the smokers around 19% and the side-stream smokers 20%."
A similar trend was seen with the implantation rates. Implantation was successful 25% of the time among the non-smokers but only 12% of the time among smokers and side-stream smokers.
The investigators speculated that cigarette smoke might damage the egg in some way that only becomes important later in embryonic development and recommended further research.
Researchers have already shown that tobacco smoke can damage sperm.
But Simon Clark from the smokers' lobby group FOREST said: "Retrospective studies, such as this, which rely on self-reported smoking habits should be treated with extreme caution.
"Anyone who concludes from this study that so-called side-stream smoking is as potentially damaging as mainstream smoking would be seriously jumping the gun.
"The dangers of side-stream smoke would appear to be speculation and until there is clear scientific evidence to support such claims, people should be careful not to draw ill-judged conclusions."
Dr Richard Kennedy, gynaecologist and spokesman for the British Fertility Society, said: "It is a small study so it would be premature to say that side-stream smoking is as damaging as smoking.
"But it is already established that smoking adversely affects IVF outcome and fertility so I am not surprised by the findings."
A spokeswoman from the British Medical Association said: "There is no safe level of smoke inhalation.
"Prospective parents should give themselves the best possible chance of conceiving and avoid smoking altogether."