Endometriomas are more frequent on the left side.

OBJECTIVE: To investigate whether asymmetry exists in the left- and right-distribution of ovarian cystic lesions in women with endometriosis.
METHODS: We evaluated operative reports of women who underwent surgical treatment of endometrioma(n = 253) from January 1999 to December 2003. We included only those cases that had not been previously operated on (n = 234). Data of all operative findings consisted of a written report and a diagram, fulfilling the revised American Fertility Society Classification of endometriosis.
RESULTS: Endometrioma was found in the left ovary in 113 women, in the right ovaryin 67, and bilaterally in 54. Left ovarian unilateral endometrioma was found more frequently (62.8%) than right endometrioma (p < 0.001, odds ratio (OR) 2.8, 95% confidence interval (CI) 1.9, 4.4). The frequencies of left and right ovarian endometrioma were compared with the expected 50% using Pearson's chi(2)-test. The results confirmed asymmetry(p = 0.001). We found 4 cases of ovarian cancer associated with left endometrioma without histological proof of transition.
CONCLUSION: Our results confirm a left lateral predisposition of endometrioma. This predisposition may be caused by the presence of the sigmoid colon in the left side of the pelvis, which decreases peritoneal fluid movement. Our findings may support the transplantation theory of the origin for endometriosis.