The Ovarian and Uterine Cycle

The ovulatory cycle causes changes in the uterus and ovaries throughout the cycle.

Ovulation Begins In The Brain

  • The communication between the brain and the ovaries is called the Hypothalamus-Pituitary-Ovarian-Axis

  • The process begins in the hypothalamus, where gonadotropin releasing hormone (GnRH) stimulates the anterior pituitary gland

  • The pituitary releases a hormone called follicle stimulating hormone, or FSH, causing FSH levels to rise.

The Ovarian Cycle

  • High levels of FSH tell the developing ovarian follicles to secrete estrogen

  • Estrogen rising makes FSH levels fall

  • Back in the brain, high levels of estrogen and now low FSH levels tell the hypothalamus-pituitary axis to release luteinizing hormone (LH)

  • LH causes the mature follicle to burst open & the egg is released! This is ovulation.

  • After ovulation, the follicle (fluid sac that held the egg) inverts on itself, creating a temporary endocrine organ called the corpus luteum, and releases progesterone

  • Progesterone inhibits both FSH & LH - telling the hypothalamus-pituitary to stop releasing them for the rest of the cycle. This prevents ovulation from occurring again in the same cycle.

  • ~2 weeks later, If the egg is unfertilized, the corpus luteum disintegrates, progesterone levels fall, and the menstrual cycle concludes with the bleed.

The Uterine Cycle

  • The uterine lining (endometrium) undergoes major changes during the cycle

  • The uterine cycle has 3 phases

  • Menstrual: The menstrual bleed is the detachment of the uterine lining

  • Proliferative: Each cycle you make new lining when estrogen starts to rise. The uterine glands start out small and sporadic looking, growing thicker and the glands becoming more coiled as estrogen peaks

  • Secretory: At ovulation and afterwards as progesterone rises, the glands become more coiled and the lining reaches its maximum thickness. This creates the ideal environment for a fertilized egg to implant into, which is necessary for pregnancy

Hormonal communication between the brain and the ovaries is what causes changes to the ovary and uterus, and changes we observe at the cervix and vulva.

Putting It All Together

Ovarian development: what the egg or egg sac (corpus luteum) is doing

Ovarian hormones: how the brain and ovarian hormones change levels throughout the cycle

Basal body temperature: follows the path of progesterone levels, which you only make after you ovulate

Cervical fluid: shows how the cervix mucus plug dissolves and how cervical fluid is produced when estrogen levels are high in the fertile window leading up to ovulation

Cervical position: the relative position and texture of the cervix in the vaginal canal as it responds to the hormone estrogen during the fertile window leading up to ovulation

Uterine lining: shows the shedding and regrowth of the uterine lining after menstruation, approaching ovulation, and during the luteal (secretory) phases