Is 28 Days REALLY the Length Your Menstrual Cycle Should Be?

We’ve been lead to believe a healthy menstrual cycle is 28 days. But the truth is, not all menstrual cycle lengths are the same from month to month and from woman to woman. Meaning your cycle may average between 30-32 days each month, and your best friend’s may average between 21 and 23. And absolutely nothing is wrong with either. 

The “average” 28 day cycle was born from the contraceptive pill. The pill was designed to mimic a woman’s natural cycle and the nice average number of 28 days, or four weeks, was chosen. The pill had to be marketed to “regulate menstruation” (which if you have been following my blog posts you know isn’t possible), since contraception was illegal.

Your menstrual cycle begins on your first day of heavy bleeding (not spotting) and ends the day before the start of your next period.

So how long is a “normal” menstrual cycle? The length of your menstrual cycle is determined by the sum of it's three main phases:

  1. The Follicular Phase (which can last 7 to 21 days)
  2. Ovulation (1 day)
  3. The Luteal Phase (10 to 16 days)

Let’s take a look at each phase of your cycle..

Phase One: The Follicular Phase

The follicular phase begins on day 1 of your cycle (the first day of bleeding) and lasts until ovulation. During the follicular phase about 6 to 8 of your ovarian follicles enter the final phase of their journey to ovulation (the complete journey is about 100 days). This is kicked off by the hypothalamus sensing low hormone levels in the blood and releasing Gonadatropin Releasing Hormone (GnRH) to the pituitary. The pituitary then releases follicle stimulating hormone (FSH) and luteinizing hormone (LH) to the ovaries. Follicle maturation is stimulated by the release of follicle stimulating hormone (FSH).

As the follicles grow and mature they release estrogen. Estrogen levels gradually rise during the follicular phase, causing FSH levels to decrease. As estrogen levels increase, the production of LH is stimulated (LH production is NOT stimulated with low estrogen levels). At the end of the follicular phase, when estrogen and GnRD levels are high they cause a massive surge in LH, which triggers ovulation.

The most mature follicle (sometimes, but rarely, two follicles) will reach ovulation.

The length of the follicular phase can be influenced by:

  • The amount of FSH your pituitary produces. The younger you are the less FSH you produce, resulting in a longer follicular phase. As you age you produce more FSH, resulting in a shorter follicular phase (common during perimenopause). A teenager can have a follicular phase as long as 32 days. 
  • Stress, Illness, Undereating or Polycystic Ovarian Syndrome (PCOS) can all cause a long follicular phase, if you are younger than 45
  • Thyroid Disease, PCOS, or the transition to menopause for those older than 45 can cause a longer follicular phase


Phase Two: Ovulation

As mentioned above, a single dominant follicle will reach ovulation at the end of the follicular phase. This follicle will swell and then burst to release an egg, triggered by the surge of luteinizing hormone.

The swelling of the follicle takes a few hours and the rupture of the egg lasts only a few minutes. The exact day on which ovulation occurs depends on the length of your follicular phase.

The egg that has been released is then transported into one of your fallopian tubes, where it has the potential to be fertilized (if sperm is present).

The other follicles (the remaining 5 to 7) are reabsorbed by your ovary.


Phase Three: The Luteal Phase

The luteal phase happens ONLY if you have ovulated (if you do not ovulate you have what is called an anovulatory cycle). The emptied follicle that released the egg during ovulation forms into a temporary endocrine gland called the corpus luteum. The corpus luteum forms within one day and stays alive for 10-16 days.

The corpus luteum secretes estrogen, inhibin and progesterone. Inhibin has a negative effect on your pituitary gland, and inhibits the secretion of follicle stimulating hormone (which makes sense because during this time no other follicles need to be maturing and growing). The rise of progesterone levels along with inhibin levels also inhibit the release of GnRH from the hypothalamus, causing a slow down in the production of estrogen. Towards the end of the 10-16 day period, as the corpus luteum begins to disintegrate, and there is no fertiliation of the egg, progesterone levels drop. The decline in progesterone along with other hormones no longer inhibit the secretion of GnRH from the hypothalamus, and the shedding of the endometrial lining is triggered, beginning a new cycle.

The luteal phase lasts 11-16 days, and can never last longer than 16 days. The most common cause of a short luteal phase (less than 11 days) is stress, but undereating or illness can also be culprits. 

To summarize, the length of your menstrual cycle can be affected by many things; age, stress levels, illness, and caloric intake.  As these things can vary from month to month, especially stress levels and caloric intake, it makes sense the length of your cycle would vary by a few days as well. As an adult, a healthy cycle can be anywhere from 21 to 35 days. In your teenage years, after you first get your period it is completely normal for your period to take a few years to regulate and normalize. Between your 20's to mid 40's things should stay fairly regular, for you - meaning, compare your cycle each month to the months prior, not to someone else's cycle.

Any drastic changes in the length of your cycle, or consistently long (35 days+) or short (less than 21 days) are worth investigating.