Different aetiological mechanisms for unexplained and endometriosis-associated infertility cannot be inferred from unstimulated IVF cycles using HCG to induce ovulation.

Omland and colleagues’ paper examining the outcome of IVF in a ‘natural cycle’ model to study aetiological differences between endometriosis and other infertility groups interested us (Omland et al., 2001). Their findings differ from ours and we have tried to understand these differences. It is important to distinguish between this study, which used HCG as a surrogate for the endogenous LH surge when fixed arbitrary criteria were reached, and truly natural cycle programmes, such as ours, devoid of all drugs. Our own programme was designed as a model for studying the mechanisms underlying unexplained and minor endometriosis-associated infertility in completely natural cycles. We found significantly longer follicular phase, significantly reduced LH surges, significantly lowered LH-Area under the curve (AUC) and reduced fertilization rates in women with minor endometriosis compared with prolonged unexplained or tubal infertility (Cahill et al., 1995, 1997). Omland and colleagues found no differences in AUC-LH or peak LH concentrations. There are however profound differences in the methodologies of the studies that we feel this largely explains their failure to find any difference. Our serum samples were undertaken every 4 hours; theirs was ‘daily or every other day’. We suggest that our determination of the AUC-LH and peak LH measurements was therefore more rigorous and sensitive in determining differences.