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Understanding Menstrual Cycle

The average adult menstrual cycle lasts 28-35 days and is divided into two phases:

  • The folliular phase begins at menses (first day of your period) and ends on the day of the LH (lutenizing hormone) surge.
  • The luteal phase begins on the day of the LH surge and ends the day prior to your next menses (period).

Menstrual cycles vary among women.  Some women are extremely regular and know exactly when to expect their periods.  Others are extremely irregular and may go multiple months without menstruating at all.  The heaviness and duration of the flow as well as pre-menstrual symptoms also vary among women and often even from cycle to cycle in the same woman.  Pre-menstrual symptoms occur due to changes in the concentrations of the hormones estrogen and progesterone and often include:

  • Breast tenderness
  • Abdominal/uterine cramps
  • Mood swings
  • Headaches

Many women think of their menstrual cycle as their period, but menstruation is actually only a very small part of a very complex cycle involving many hormones secreted by both the pituitary gland and the ovary.

During the early follicular phase, the FSH (follicle stimulating hormone) increases by about 30%.  This increase appears to be required for stimulating the next group of ovarian follicles to develop.  One of these developing follicles will eventually be released into the fallopian tube at ovulation.  The endometrium is very indistinct during menstruation.

How you might be feeling: you may continue to have some of the pre-menstrual symptoms for the first few days as your hormones are attempting to stabilize.  You are shedding your endometrial lining (your period), so you will be using pads or tampons.

During the mid-follicular phase, the increase in FSH that occurred during the first phase causes the follicles to continue to develop and produce estrogen, which causes the endometrium to begin to thicken.

How you might be feeling: menstruation will have stopped; any pre-menstrual symptoms that you might have had should be resolved, so you may be feeling pretty good.

During the late follicular phase, which is the week leading up to ovulation, the serum estrogen concentration increases daily.  This causes the continued thickening of the endometrium, which is important because the egg will need a healthy blood-rich area in which to implant if it becomes fertilized.  Out of the group of developing follicles, one has become dominant and will continue to grow and develop while the others will recede. 

How you might be feeling: you may notice an increase in the amount and stringiness of your cervical mucus.  It is often described as being the consistency of egg whites and is the result of the increased estrogen.  It is considered fertile cervical mucus, so if you are trying to conceive, now is a good time to have intercourse.

The early luteal phase is essentially the mid-cycle LH surge followed by ovulation.  LH pulses are continuing at a rate of one per hour, but the amplitude of each pulse increases significantly.  The oocyte (egg) is released from the follicle approximately 36 hours after the surge and then travels down the fallopian tube and into the uterine cavity.

How you might be feeling: some women experience breast tenderness or  mild abdominal cramping around ovulation.  Many women will continue to have egg white cervical mucus until after ovulation.  If you are trying to conceive, you can detect your LH surge with ovulation prediction kits, which helps you to have intercourse during your most fertile time.

Progesterone is released by the corpus luteum soon after ovulation in the mid-luteal phase.  This rise leads to a slowing of the LH pulses.

How you might be feeling: if you are temping in an effort to conceive or avoid conceiving, you should notice an increase in your basal body temperature approximately 24 hours after ovulation.  This thermal shift is due to the increase in progesterone. 

In the late-luteal phase, there is a decrease in LH, which leads to decreasing levels of both estrogen and progesterone, which leads to a decrease in the endometrial blood supply, which causes the endometrial lining to shed (your period).  However, if the egg was fertilized and successfully implanted, it will produce the hormone hcg (human chorionic gonadotropin), which maintains the corpus luteum and progesterone production until the placenta develops and takes over.

How you might be feeling: you may have tender breasts, abdominal or uterine cramping, mood swings, or headaches.  These pre-menstrual symptoms can be caused by the high levels of progesterone and can be mistaken for early pregnancy symptoms, which are also caused by elevated progesterone.

So instead of being annoyed at the inconvenience of your monthly period, try to be appreciative of the complexity of the cycle that is central to the miracle of life!