<P> Female fertility is affected by age . Age is thus a major fertility factor for women . Menarche, the first menstrual period, usually occurs around 12 - 13, although it may happen earlier or later, depending on each girl . 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, this is unclear since studies are still cited from the nineteenth century and earlier . One 2004 study of European women found fertility of the 27 - 34 and the 35--39 groups had only a four - percent difference . At age 45, a woman starting to try to conceive will have no live birth in 50--80 percent of cases . 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 . The relationship between age and female fertility is sometimes referred to as a woman's "biological clock ." </P> <P> The average age of a young woman's first period (menarche) is 12 to 13 (12.5 years in the United States, 12.72 in Canada, 12.9 in the UK) but, in postmenarchal girls, about 80% of the cycles are anovulatory in the first year after menarche, 50% in the third and 10% in the sixth year . A woman's fertility peaks in her early and mid-20s after which it starts to decline . However, the exact estimates of the chances of a woman to conceive after a certain age are not clear, and are subject to debate . </P>

When does a woman's biological clock stop