Menstrual Cycle

 

Menarche is the beginning of the menstrual cycle.

It begins with hypothalamic secretion of GnRH, and other hormones, leading to adrenal androgen release and pituitary growth hormone.

GH causes the growth spurt a few years before menarche, and growth slows down as menarche begins. This is because increasing levels of estrogen, released by the ovaries, provides negative regulation of GH.

The hypothalamus begins secreting GnRH in episodic pulses of 2 hour intervals. This causes the release of FSH and LH.

Estrogen is the principle hormone secreted until age 8, though only in small quantities. After that, estrogen levels rise slowly and then more repidly after age 11. FSH plateaus at about age 13, while LH rises more slowly until a year before menarche. After that, a rapid rise occurs.

GnRH pulses ever 90 minutes, a pattern that contuines until age 40 and the beginning of menopause.

 

We believe the rise in LH is important in the onset of menarche, along with increased fat:lean body mass.

Menarche now occurs at age 12-13, earlier than age 15 a century ago. This is thought to be related to better childhood nutrition and fat stores, which mediate the transformation of androgen to estrogen. Rapidly rising estrogen levels lead to the LH surge.

Menarche can be delayed in people with poorer fat stores, such as athletes, people with anorxeia, or those in poorer health.

 

 

 

 

G