BACKGROUND It is not uncommon for women with endometriosis to be advised that becoming pregnant might be a useful strategy to manage their symptoms and reduce disease progression. Consequently, many women diagnosed with endometriosis and motivated to become pregnant, may also have expectations regarding improvement of symptoms and the disease. However, study results on the effect of pregnancy on endometriosis are controversial and pregnancy in women with endometriosis is not always associated with improved symptoms. Moreover, there is increasing evidence that endometriosis may interfere with a successful pregnancy outcome. OBJECTIVE AND RATIONALE The objective was to evaluate the evidence on whether pregnancy and lactation has a beneficiary effect on growth characteristics and symptoms of endometriosis diagnosed prior to pregnancy. SEARCH METHODS A search for articles containing keywords related to pregnancy and endometriosis was performed via PubMed. Manuscripts dealing with a potential effect of pregnancy on endometriosis were systematically reviewed. We included English, French and German language publications on human studies from 1966 to May 2017. Bibliographies of these manuscripts were searched for further relevant literature. OUTCOMES Five small observational studies were identified concerning the longitudinal development of endometriotic lesions during and after pregnancy, four of medium and one of low quality. Eleven publications reported measurements of endometriomas during pregnancy and the postpartum period (the five studies just mentioned and six case reports). Another 22 case reports/small case series (maximum of five cases), six studies on histology of endometriotic lesions in pregnancy, plus eight studies on the role of pregnancy in initial development and recurrence of endometriosis were included. Few studies of very limited quality are available to evaluate the effect of pregnancy and the postnatal period on the development of endometriosis. The development of endometriosis is variable and there is no evidence that pregnancy can be expected to generally reduce the size and number of endometriotic lesions. Growth and structural changes of lesions during pregnancy may occur with decidualization. Results on the association between pregnancy and symptoms of endometriosis are controversial and strongly biased. WIDER IMPLICATIONS Available data on the development of endometriosis during and after pregnancy show fewer beneficial effects than previously reported. Therefore, women aiming for pregnancy on the background of endometriosis should not be told that pregnancy may be a strategy for managing symptoms and reducing progression of the disease.