menstrual (estrous) cycle

Physiology

-..........-

Day   event
0     menses
2 ... day 2: begin follicular phase
6
10
.....- day 13: estradiol peak
14 ... day 14: FSH & LH surge, begin luteal phase, progesterone secretion
20
24
28 ... menses

GnRH:

• increased frequency of GnRH pulsatile secretion early in follicular phase
• abrupt change in frequency of GnRH pulses during late luteal phase as levels of sex steroids & inhibin A secretion wane
• lowest levels of GnRH 10 days after mid-cycle LH surge

Estradiol:

• mid-cycle surge
  • begins day 8 or 9
  • peaks at day 13
  • minimum at day 16
• secondary rise & plateau during luteal phase in parallel with progesterone

Follicular phase:

• dominant follicle development is coordinated with atresia of remaining follicles
• dominant follicle reaches size of 1.5-2.5 cm at the time of ovulation
• gradual decline in FSH as follicular phase progresses
• rising estradiol secreted by dominant follicle evokes LH surge from pituitary
• LH surge ruptures dominant follicle, releasing mature oocyte

Luteal phase:

• corpus luteum forms from remnants of ruptured follicle
• corpus luteum secretes progesterone, estradiol & inhibin A resulting in slowed GnRH pulse
• temporary predominance of FSH over LH for 3-7 days in late luteal phase that recruits a new wave of folliculogenesis in the ovaries

Clinical significance

• normal length 25-35 days
• in women < 40 years of age, menstrual cycle < 25 days or > 35 days likely anovulatory

More general terms

• physiologic process

Additional terms

• menopause
• menorrhagia
• menstrual disorder
• menstruation (menses, menstrual period)

Components

• luteolysis