This is defined as inability to conceive for 12 months during which approximately 80%–90% of couples desirous of pregnancy will have done so. The prevalence is 14%, at least half of couples complaining of secondary infertility. The infertility will remain unresolved in 3%–4% of couples. The mean time to pregnancy rises from six months in women at all ages attempting their first pregnancy to 15 months in a 35-year-old undergoing artificial insemination with donor sperm (DI) or more than seven years if, in addition, this patient has been treated for endometriosis. The monthly probability of pregnancy (fecundability) expressed as the reciprocal of the mean time required for conception in months falls from 0.4 in the 25 year-old attempting her first pregnancy to 0.008 after two years of unexplained infertility.