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NEW YORK (Reuters Health) - Smoking may neither raise nor lower a woman's chances of developing endometriosis, one of the most common causes of infertility, a new study suggests.Endometriosis is a disorder in which pieces of the tissue that lines the uterus (the endometrium) grow outside of the uterus, most often on the ovaries, fallopian tubes, surface of the uterus or lining of the pelvis. This displaced tissue continues to act like the uterine lining -- responding to the menstrual cycle by thickening, then breaking down and bleeding each month. That can lead to the formation of scar tissue, causing symptoms like pelvic pain, heavy menstrual bleeding and, in some women, infertility.What causes endometriosis remains a mystery, but estrogen is involved in triggering the monthly buildup and breakdown of endometrial tissue. Because smoking tends to lower a woman's estrogen levels, researchers have speculated that the habit might lower the risk of developing endometriosis, or at least more-severe endometriosis.But in the new study, French researchers found that among nearly 1,000 women with and without endometriosis, there was no clear link between current or past smoking and the risk of having the disorder. The findings remained the same when the researchers separated the women into groups based on endometriosis severity.Researchers led by Dr. Charles Chapron, of Cochin-Saint Vincent de Paul Hospital in Paris, report the findings in the journal Fertility and Sterility.The study included 978 women younger than 42 who had been evaluated for endometriosis; 411 were confirmed as having the disorder and had tissue samples examined to determine the severity of the condition. Of the 411 women with endometriosis, 45 percent were current or former smokers, as were 36 percent of the 567 without endometriosis. A first look at the data suggested that smoking was linked to an increased risk of endometriosis.But that connection disappeared when Chapron and his colleagues accounted for other factors, including age, weight and whether a woman had ever had children. (Women who have never had children appear to have higher rates of endometriosis than those who have given birth. And some studies have found that thinner girls and women may have a higher risk than their heavier counterparts.) Nor was there any evidence that smoking affected the risk of having more-severe, extensive endometriosis.The study has its limitations, including relying on women's reports of their smoking history. However, the researchers say, that is unlikely to explain the lack of a relationship between smoking and endometriosis risk. For now, they conclude, the findings suggest that smoking does not, by itself, influence the risk of endometriosis.