NEW YORK, Sep 30 (Reuters) Women who exercise routinely for four or more hours per week may reduce their chances of having a successful pregnancy with in vitro fertilization (IVF), new research suggests.

''Although exercise has many known health benefits, it does not seem to contribute to successful IVF outcomes,'' senior author Dr Mark D Hornstein, from Brigham and Women's Hospital in Boston, and colleagues note in the journal Obstetrics&Gynecology.

''However,'' they add, ''our findings are not strong enough to encourage women to abandon exercise and embrace a sedentary lifestyle.'' The researchers assessed various IVF outcomes in 2232 women who underwent their first IVF cycle between 1994 and 2003 in the greater Boston area.

In general, regular exercise did not seem to decrease or increase the chances of having a baby through IVF, the report indicates.

However, with four or more hours of exercise per week for 1 to 9 years, the odds of a live birth fell by 40 per cent relative to engaging in no exercise.

In addition, this level of activity raised the risks of implantation failure and pregnancy loss.

The team also looked at the potential association of a woman's body mass index (BMI) with success rates, because of the increased risk of infertility in both underweight and overweight patients.

''However, we did not observe any difference in the relation between exercise and IVF outcomes among the different BMI groups,'' they write.

Further research is needed to confirm and expand on these findings, the investigators conclude, emphasizing that it is too early to make definitive recommendations.

REUTERS MQA PM0927