The average age of a young woman's first period (menarche) is 12 to 13 (12.5 years in the United States,[8] 12.72 in Canada,[9] 12.9 in the UK[10]) 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.[11] 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.[12]