Female Menstrual Cycle
During women’s reproductive years, our hormones should be fluctuating throughout the month. These hormonal fluctuations are orchestrated between glands in the brain and the ovaries called the hypothalamic-pituitary-ovarian axis (HPG axis). Their purpose is to release eggs for possible fertilization and prepares the uterus for possible implantation. As we dive into some common women’s health complaints, it is important to first understand what should be occurring throughout the monthly cycle.
The follicular phase of the menstrual cycle begins on the first day of menses. Declining hormone production at the end of the previous cycle, allows for the release of LH and FSH from the anterior pituitary in the brain. These hormones then travel to the ovaries. The elevation in FSH, stimulates the growth of about 15-20 eggs, each in their own follicle. FSH and LH also increase the production of estrogen in the ovary. As estrogen levels rise during this phase, it eventually inhibits the secretion of FSH. This feedback loop helps to limit the number of follicles that will prepare eggs for release.
As the follicular phase progresses, only one follicle will become dominant and continue to mature. The other follicles will undergo atresia, where they stop growing and die. The dominant follicle will continue to produce estrogen. While increasing estrogen during the follicular phase inhibits FSH release from the pituitary, it stimulates the release of LH. While the mature follicle is developing in the ovary and estrogen levels are rising, the uterine lining is starting to proliferate (called the proliferative phase).
SUMMARY MENSES: Both estrogen and progesterone are low.
SUMMARY FOLLICULAR PHASE: FSH stimulates follicle growth. Estrogen levels rise until end of the follicular phase.
Ovulation usually occurs around 14 days after the start of the follicular phase (in a 28 day cycle), although this can vary. The continued rise in estrogen from the dominant follicle, will trigger a surge in LH release from the pituitary. This LH surge is what is responsible for the follicle rupturing and releasing its egg. The high LH levels stimulate increase intrafollicular enzymes that weaken the wall of the ovary so that the follicle can pass through. Ovulation will occur approximately 24-36 hours after the peak in LH levels. LH also stimulates the transformation of the ruptured follicle into the corpus luteum, which will produce progesterone during the next phase of the cycle.
SUMMARY OVULATION: High estrogen stimulates LH surge. Ovulation occurs about 12 hours after LH peaks.
The luteal phase begins right after ovulation. The corpus luteum is formed by transforming the ruptured follicle. It secretes primarily progesterone and some estrogen, and its primary function is to prepare the estrogen-primed uterine lining for implantation of a fertilized egg. Higher progesterone levels also cause cervical mucus to thicken and increase body temperatures slightly. The higher levels of estrogen and progesterone seen in the luteal phase, inhibit LH and FSH secretion from the pituitary, so the corpus luteum’s function will decline toward the end of the luteal phase unless human chorionic gonadotropin (hCG) is produced in response to a pregnancy. If a sperm has fertilized the egg, the egg will travel through the fallopian tubes to implant in the uterus. The implanted egg will produce human chorionic gonadotropic (hCG) and will maintain corpus luteum function and estrogen/progesterone levels. If there is no pregnancy/rise in hCG, the corpus luteum will decline to function around 14 days after ovulation. The failure of the corpus luteum causes a drop in estrogen and progesterone. This drop in hormones in combination with prostaglandins will start menses and the next menstrual cycle.
SUMMARY LUTEAL PHASE: Drop in LH & FSH. Progesterone is at its highest