After puberty, female fertility increases and then decreases, with advanced maternal age causing an increased risk of female infertility. In humans, a woman's fertility peaks in the early and mid-20s, after which it starts to decline slowly. While many sources suggest a more dramatic drop at around 35,[1] this is unclear since studies are still cited from the nineteenth century.[2] One 2004 study of European women found fertility of the 27-34 and the 35–39 groups had only a four-percent difference.[3] At age 45, a woman starting to try to conceive will have no live birth in 50–80 percent of cases.[4] Menopause, or the cessation of menstrual periods, generally occurs in the 40s and 50s and marks the cessation of fertility, although age-related infertility can occur before then.[5] The relationship between age and female fertility is sometimes referred to as a woman's "biological clock."[6]