Anovulatory cycles are most common in the first and last few years of reproductive life, when hormone levels are less stable. In the first years after menarche, anovulatory cycles are in fact the norm. Attainment of menarche has been hypothesized to require a threshold percentage of body fat of around 17% (Frisch, 1988; Frisch & McArthur, 1974) and in North America, the average age at menarche is about 12.5 years (Bullough, 1981). The advent of menstrual cycling does not imply full ovulatory competence and cycles in the first few years tend to be irregular

Anovulatory cycles are most common in the first and
last few years of reproductive life, when hormone
levels are less stable. In the first years after menarche,
anovulatory cycles are in fact the norm. Attainment of
menarche has been hypothesized to require a threshold
percentage of body fat of around 17% (Frisch,
1988; Frisch & McArthur, 1974) and in North
America, the average age at menarche is about 12.5
years (Bullough, 1981). The advent of menstrual cycling
does not imply full ovulatory competence and
cycles in the first few years tend to be irregular