In this prospective cohort study, 30 women diagnosed at laparoscopy with mild endometriosis were treated expectantly and followed for 10 years. Eleven women had primary infertility and 19 secondary infertility. Before the diagnosis of endometriosis, 19 women had a total of 32 pregnancies: the female live birth rate was 25%, male live birth rate 28%, spontaneous abortion rate 44%, and ectopic pregnancy rate 3%. After expectant treatment of mild endometriosis, 25 women had a total of 51 pregnancies: the female live birth rate was 52.9%, male live birth rate 27.4%, spontaneous abortion rate 19.6%, and ectopic pregnancy rate zero. Expectant treatment of mild endometriosis in 30 infertile women resulted in 41 live-born infants: 66% females and 34% males. We conclude that expectant treatment of mild endometriosis selectively favors the survival of female infants.