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Introduction

The menstrual cycle involves a complex interplay of hormones controlled by the hypothalamic-pituitary-ovarian (HPO) axis to regulate the ovary and uterine endometrium.  The menstrual cycle is approximately 28 days in length and consists of 2 main phases—follicular and luteal—separated by ovulation.

Hypothalamic-pituitary-ovarian axis

The HPO axis is the central regulatory system of the menstrual cycle and consists of the following components ( Figure 1):

  • Hypothalamus:  Releases pulsatile gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland.
  • Anterior pituitary gland:  Releases gonadotropins (follicle-stimulating hormone [FSH], luteinizing hormone [LH]) in response to GnRH.
  • Ovary:  Responds to FSH by initiating follicular development, which produces estradiol through follicular steroidogenesis.

Follicular phase (days 1-14)

The follicular phase begins on the first day of menstruation (

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