The harmful effects of smoking or being overweight were strongest among those women who had no obvious cause for not conceiving, according to the research, published today (Thursday 7 April) in Europe's leading reproductive medicine journal Human Reproduction.
Lead researcher Dr Bea Lintsen, from Radboud University Nijmegen Medical Centre, said: "The positive news from our results, however, is that they suggest that couples ?in particular, women with unexplained subfertility ?may be able to improve the success of IVF treatment by quitting smoking and losing weight."
The research was a collaboration by doctors from twelve centres in the Netherlands who drew on data from a proportion of patients involved in the nation-wide OMEGA study, which was begun in 1995 to examine late effects of hormone stimulation. The team investigated the success rate of the first cycle of treatment in 8,457 women, analysing information on reproduction and lifestyle factors, combined with medical records of IVF treatment from 1983 to 1995. ICSI cycles were not evaluated as the treatment was not common during that period.
They divided the cause of subfertility into four categories ?fallopian tube problems, male subfertility, unexplained fertility, and other causes (mainly women with polycystic ovarian syndrome or endometriosis) ?and analysed first cycle treatment results for the first three causes. Of the total, 1,828 of the first IVF treatment cycles were for unexplained subfertility.
The overall live birth rate per cycle was 15.2%. It was highest at 17.8% in couples with unexplained subfertility. Women with tubal problems had a 14.6% rate and the lowest success rate was for th ose with male subfertility (13.6%).
More than 40% of the women were smokers at the time of undergoing their first attempt at IVF and more than 7% were overweight i.e. had a body mass of 27kg/m2 or more.
Older women had lower success rates no matter what was the cause of subfertility, with the live birth rate decreasing by 2% for each additional year of age.
"In all subgroups smokers had a lower delivery rate per treatment cycle than non-smokers," Dr Lintsen said. "The live birth rate for smokers was 28% lower than that of non-smokers. This was most marked in women who had no known cause for subfertility where the live birth rate among smokers was only 13% compared to 20% for non-smokers."
The miscarriage rate per pregnancy was also significantly higher for women who smoked, with around 21% losing their babies compared with around 16% of non-smokers.
Overweight women also fared badly with a 33% lower chance of a live birth from their first treatment cycle. As with smoking, the unfavourable effect of overweight was largest in women with unexplained subfertility. Only 14% had a live baby compared with over 18% for normal weight and nearly 21% for slightly overweight women.
Professor Didi Braat, one of the co-authors, said: "What our research clearly shows is that both smoking and being overweight unfavourably affect the live birth rate after IVF. Smoking has a devastating impact. It is comparable to adding a decade to the reproductive age of a 20-year-old. This means it makes her the equivalent of a 30-year-old non-smoker in reproductive terms.
"However, it also indicates that subfertile couples may help their chances of successful treatment by life-style changes. As the effects of smoking and overweight were greatest among women with unexplained subfertility, these results suggest that this group in particular may be able to improve the outcome of subfertility treatment by quitting smoking and losing weight."
 Effects of subfertility cause, smoking and body weight on the success rate of IVF. Human Reproduction. Doi:10.1093/humrep/deh898
 The authors point out that the success rates from this data might differ from success rates today, as success rates have increased since 1995 when the OMEGA study began.