Class Is In Session: A Breakdown of the Menstrual Cycle

The Menstrual cycle is divided into two events and two phases:

  • Menses (your “period”)
  • Follicular phase (phase 1)
  • Ovulation
  • Luteal phase (phase 2)

MENSES (“PERIOD”)

  • This is DAY 1 of your new cycle.
  • It is the shedding of the uterine lining (endometrium) that grew during your previous cycle.
  • All hormone levels are low.

FOLLICULAR PHASE

  • Begins on day 1 of your new cycle (1st day of your period).
  • Defines the first half of the cycle (roughly the first 14 days), from menses to ovulation.
  • FSH (Follicle Stimulating Hormone), produced by the pituitary gland in the brain, triggers
    follicles in the ovaries to produce Estrogen.
  • This phase is dominated by Estrogen. What does Estrogen do?
    •  causes eggs to mature (so they’re ready for ovulation).
    • builds uterine lining (for implantation of a fertilized egg).
    • makes fertile quality cervical fluid (for sperm to travel through to fertilize the egg).
  • Right before ovulation (theoretically day 13 in a 28-day cycle), cervical fluid quality and estrogen levels are at their peak, and the cervix is high and open.
  • At the end of the follicular phase, high estrogen levels trigger a surge of LH (Luteinizing Hormone) from the pituitary gland which completes the growth of the follicle (containing the egg) and stimulates ovulation.

OVULATION

  • The egg is released from the follicle within the ovary and is transported to the Fallopian tube.
  • The egg survives for 24 hours after ovulation.
  • Date of ovulation determines your cycle length.
  • Ovulation does not always occur on day 14.
  • Ovulatory bleeding is normal, which looks like mid-cycle spotting.
  • Ovulation is indicated by a subtle increase in basal body temperature (BBT).
  • Multiple ovulation (2 eggs released in 24 hours) can occur, this is more common in older women.

LUTEAL PHASE

  • Defines the second half of the cycle, between ovulation and menses.
  • Length stays fairly consistent each cycle, lasting between 12-16 days (average is 13, 14). Maximum is 16 days.
  • This phase must last at least 10 days for embryo implantation into the uterine lining.
  • The length of this phase is not affected by external factors (i.e-stress), therefore a late
    period is usually due to delayed ovulation.
  • The follicle that housed the released egg turns into the “corpus luteum” and produces progesterone.
  • This phase is dominated by Progesterone. What does Progesterone do?
    • keeps the lining of the uterus thick in case an embryo implants.
    • produces body heat (reason for increased Body Basal Temperature post-ovulation).
    • causes changes in cervical fluid and position.
    • prevents release of another egg during the same cycle.

What happens if a sperm fertilizes the egg?

If a sperm fertilizes the egg within 24 hours after ovulation, then 7 days later the “blastocyst” (the stage between fertilized egg and embryo) implants in the uterine wall. The blastocyst produces the hormone “HCG” (Human chorionic gonadotropin), a pregnancy test measures this hormone. HCG causes the corpus luteum to live past 16 days and continue producing progesterone until the placenta takes over to sustain fetal development (around week 10 of pregnancy).

What happens if the egg is not fertilized?

The corpus luteum dies (after 12-16 days) and stops making progesterone. Without progesterone, the uterine lining is no longer being maintained which causes it to shed…. you get your period (menses), and the cycle starts all over again!

Article by Dr. Sara Schwerd