The measurement of fertility is also used to assess the effect of treatment. Although the occurrence of pregnancy may seem an objective criterion, it has several characteristics that make its use as an outcome variable problematic. First, multiple confounding factors often affect a woman’s fertility. In addition, pregnancy (like pain) is a time-dependent phenomenon. That is, the pregnancy rate will vary depending on the duration of follow-up. Finally, endometriosis rarely causes an absolute inability to conceive; thus, the rate of pregnancy in untreated women must be used as a basis for comparison