Why is My Menstrual Cycle Getting Shorter?

Dr. Jolene BrightenPublished: Last Reviewed: Menstrual Cycle Leave a Comment

Some women get their period like clockwork, following the same pattern each month. For others, the menstrual cycle can vary by a few days each month. And for most women, cycle length gradually decreases with age. All of these patterns are completely normal. 

However, if you notice your menstrual cycle is consistently getting shorter, or your typical 28-day cycle suddenly drops to 21 days, you may be wondering why is my menstrual cycle getting shorter?  It’s worth taking another look at why this is happening. 

Your menstrual cycle provides a glimpse into what’s happening internally with your hormones, which translates to your whole-body health. Let’s look at why your menstrual cycle may be getting shorter and when you should consider reaching out for additional support.

How Long Should a Menstrual Cycle Be?

To start, let’s look at what a typical menstrual cycle looks like. Whenever I talk about a “normal” menstrual cycle, I always do so with the reminder that since we are all different, there is a range of what is considered normal. Cycle length can vary between women and even change monthly within your own cycle. 

We often hear the average cycle length is about 28 days, but very few people actually have a 28 day cycle. So we’ll often say, it’s about 28 days, give or take three to five days. Your menstrual cycle is the number of days between periods. So day one of your cycle starts on the first day of your period and the last day of your cycle is the day before your next period begins. 

@drjolenebrighten It’s normal. How many days is your cycle? #period #periodproblem #menstrualcycle #ovulationmatters #bodyliteracy #latinxcreatives #familialatina #hormoneimbalance #hormones #periodpositive #healthyliving ♬ TWINNEM – Coi Leray

Your cycle is broken up into four phases:  

  • Menstrual phase
  • Follicular phase
  • Ovulation phase
  • Luteal phase

Estrogen rises and your uterus lining thickens during your follicular phase as your body prepares for ovulation. Ovulation is the release of the egg from your ovary and is usually the midway point of your cycle. 

If no pregnancy occurs during ovulation, the uterine lining sheds following your luteal phase, resulting in your period. 

All these critical steps are regulated by your hormones, especially estrogen and progesterone. As a result, anything that disrupts hormonal balance can trigger changes in your cycle.

Menstrual Cycle Length and Age

If your menstrual cycle is getting shorter, one of the first things to consider is your age. From the early days after you first start your period to the years leading up to menopause, it’s normal to experience fluctuations in cycle length. 

Early menstrual cycles are often irregular in the teen years as hormones adjust and mature. Around age twenty, cycle length tends to be more regular and predictable as long as there’s no illness or other underlying condition.

By your mid-twenties, your cycle will start changing very gradually. One study found that mean cycle length decreased by 0.18 days per year between age twenty-five and forty-five. The change is so subtle you won’t really notice it, but as you get closer to perimenopause, it can become more noticeable.

Perimenopause is the transition before menopause and can last for several years. You may notice shortening cycles with heavier periods during this time. 

Research suggests that women over forty often have cycles with longer luteal phases and a shortened follicular phase. One study that followed a group of women between the ages of 42 and 52 for three years found that cycles shorter than 21 days were common in early perimenopause and associated with not ovulating (anovulation). 

As you get closer to menopause, cycles may lengthen or alternate between shorter and longer, becoming much more erratic. Eventually, your period stops coming as you no longer ovulate.

Reasons for a Short Menstrual Cycle

Aside from age, a short menstrual cycle can occur for several reasons. Cycle length may shorten because your period only lasts a few days. Or it can be due to your period actually coming earlier than usual.

Reasons for a Short Period

A short period means you bleed for fewer days than what’s considered normal. Most women bleed for three to five days. 

While variations are possible, a short period may be a sign that something’s amiss with your hormone balance, especially if the length of your period has changed compared to your usual patterns.

There are various reasons this can happen:

  • Breastfeeding. Prolactin, the hormone that helps you produce breastmilk, interferes with regular menstruation. It’s part of why women who breastfeed don’t get their period back right away. Even when it comes back, your period can be lighter or shorter than usual. As your baby nurses less often and milk production drops, your period should come back as before.
  • Pregnancy. If you have a short or much lighter period than usual, it could be due to something called implantation bleeding. Implantation bleeding occurs when the fertilized egg implants in your uterus. While this isn’t an actual period, implantation bleeds can be confusing because they can last a day or two, so you may think you are having a short period. 
  • Birth Control Pill. I need to preface this by saying that your bleed is not an actual period if you are on a birth control pill. Or even right after you stop taking it. It’s known as a withdrawal bleed because when you stop taking the hormones found in your pill, the sudden drop in hormones causes the lining of your uterus to shed. That said, many women notice shorter bleeds, if any, while on the pill.
  • Medications. Aside from the pill, certain medications may make your period shorter or lighter, like ibuprofen or anti-clotting drugs. These are often prescribed to women with heavy periods to reduce blood loss (but don’t address the root cause).
  • Stress. Since it’s well-known that chronic stress can wreak havoc on our health, it’s no surprise that it can also mess with our hormones. Stress can lower estrogen and progesterone and delay or completely stop ovulation, leading to short periods. 
  • Thyroid. If your periods are suddenly wonky, look at your thyroid. Thyroid hormone is critical for healthy egg development and signalling between your brain and ovaries. As a result, low thyroid hormone levels (hypothyroidism) can result in alterations to your period, including very light bleeds.
  • Over-exercise. We know movement is good for us, but too much of anything isn’t great, including working out. Like chronic stress, over-exercise can put excess stress on the body, impacting your hormones and shortening your periods.

What Can Cause Your Period to Come Early?

An early period indicates that your overall cycle is shorter. You may bleed for a typical length of time (or even an extended period), but your period comes sooner than it should. If your cycle is short, it can feel like you have a very short window between periods.

Short and early periods have many of the same root causes stemming from hormonal changes. Here are some of the reasons to consider:

  • You aren’t ovulating. Anovulatory cycles, where progesterone remains low, can result in an early period. Technically this isn’t considered a “real” period since no egg is released. Still, if your uterine lining builds up during the follicular phase, it will result in a bleed. Here’s how to know if you’re ovulating.
  • PCOS. PCOS is complex, but fundamentally it’s characterized by an imbalance of hormones that impact ovulation, leading to shorter cycles or sometimes, longer cycles. 
  • Thyroid. As noted earlier, people with thyroid dysfunction are much more likely to experience irregular menstrual cycles, including short cycles. 
  • Pregnancy. Once again, an implantation bleed could be confused with a period and make you think you are having a short cycle. 
  • Miscarriage. Trigger warning, but it should be noted that it is possible to miscarry even before you know you are pregnant, which could be confused with an early period.
  • Endometriosis. Endometriosis is the growth of endometrial-like tissue outside your uterus like fallopian tubes, ovaries, or intestines. It has a spectrum of symptoms, but heavy bleeding with shorter cycles is common.

Can a Short Cycle Affect Fertility?

Short cycles could affect your chances of getting pregnant, but not always. As a short cycle can indicate hormone imbalance or that you aren’t ovulating, it can impact your fertility. Without ovulation, you can’t get pregnant.

But even if you are ovulating, there’s a chance that a shorter cycle could impact your chances of getting pregnant. One study found that cycles shorter than 26 days were linked to lower odds of pregnancy. 

Does this mean you can’t get pregnant if your cycle is shorter?  No, but it suggests that a shorter cycle may also indicate fewer fertile days. Or, as the authors of the study listed above pointed out, shorter cycles could mean something else is going on, like anovulation or perimenopause, which both affect fertility.

If you do have a shorter cycle and have ruled out other root causes with your doctor, then there are things you can do to optimize your fertility, like eating a nutrient-dense, anti-inflammatory diet. 

Check out our free hormone-balancing plan with meal plans and recipes to get you started. Certain supplements can also support your fertility by addressing hormone balance and optimal detoxification.


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When to See a Doctor

There are many reasons for a shortening menstrual cycle, and some are entirely normal. But if you are concerned that there is something else going on, listen to your intuition. There are also reasons (as you just learned about above) that require a closer look, including PCOS or thyroid conditions. 

Your menstrual cycle is a reflection of your health as a whole, so any changes to your cycle are worth paying attention to.

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  1. R.J, Fehring. Variability in the Phases of the Menstrual Cycle. JOGNN. 2006. 35. 3.
  2. Committee Opinion. Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign. ACOG. 2020. 651.
  3. Bull, J.R., Rowland, S.P., Scherwitzl, E.B. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digital Medicine. 2019. 83.
  4. H.G Burger , G.E Hale, D.M Robertson, L Dennerstein. A review of hormonal changes during the menopausal transition: focus on findings from the Melbourne Women's Midlife Health Project. NIH. 2007.
  5. J.A. Grieger, BSc, PhD; R.J Noman, MD; Franzcog. Menstrual Cycle Length and Patterns in a Global Cohort of Women Using a Mobile Phone App: Retrospective Cohort Study. Journal of Medical Internet Research. 2020.
  6. B.J. Van Voorhis, M.D., N. Santoro, M.D., S. Harlow, Ph.D, S. L. Crawford, Ph.D J. Randolph, M.D. The Relationship of Bleeding Patterns to Daily Reproductive Hormones in Women Approaching Menopause. HHS Public Access. 2009. 101-108.
  7. J.R Taffe, L. Dennerstein. Menstrual patterns leading to the final menstrual period. NIH. 2002. 32-40.
  8. J Bonnar, M Franklin, P N Nott, and A S Mcneilly. Effect of breast-feeding on pituitary-ovarian function after childbirth. NCBI. 1975. 82-84.
  9. Bleeding During Pregnancy Frequently Asked Questions. ACOG.
  10. C. Germany. What are the treatment options for heavy periods?. Institute for Quality and Efficiency in Health Care. 2020.
  11. Schliep, Karen C.; Mumford, Sunni L.; Vladutiu, Catherine J; Ahrens, Katherine A; Perkins, Neil J; Sjaarda, Lindsey A; Kissell, Kerri A.; Prasad, Ankita; Wactawski-Wende, J. Schisterman, Enrique F. Perceived Stress, Reproductive Hormones, and Ovulatory Function A Prospective Cohort Study. Epidemiology. 2015. 26. 177-184.
  12. M. H. Jacobson, P.P. Howards, L.A. Darrow, J.W. Meadows, J.S. Kesner, J.B. Spencer, M.L. Terrell, M Marcus. Thyroid hormones and menstrual cycle function in a longitudinal cohort of premenopausal women. Paediatr Perinat Epidemio. 2018. 32. 235.
  13. O. Hakimi , L.C Cameron. Effect of Exercise on Ovulation: A Systematic Review. PubMed. 2017. 8. 1555-1567.
  14. W. Ming MD; C. Yanfei MD; B. Huaien MD; Z. Ye MD, PhD, MBA; Z. Wenli MD, PhD. Length of Menstrual Cycle and Risk of Endometriosis A Meta-Analysis of 11 Case–Control Studies. Medicine. 2016. 95. 2922.
  15. A.K. Wesselink MPH, L.A Wise SCD, E.E Hatch PhD, K.J. Rothman DrPH, E.M. Mikkelsen PhD, J.B. Stanford MD, MSPH, C.J. McKinnon MPH, S. Mahalingaiah MD,MS. Menstrual cycle characteristics and fecundability in a North American preconception cohort. Annals of Epidemiology. 2016. 26. 482-487.
About The Author

Dr. Jolene Brighten

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Dr. Jolene Brighten, NMD, is a women’s hormone expert and prominent leader in women’s medicine. As a licensed naturopathic physician who is board certified in naturopathic endocrinology, she takes an integrative approach in her clinical practice. A fierce patient advocate and completely dedicated to uncovering the root cause of hormonal imbalances, Dr. Brighten empowers women worldwide to take control of their health and their hormones. She is the best selling author of Beyond the Pill and Healing Your Body Naturally After Childbirth. Dr. Brighten is an international speaker, clinical educator, medical advisor within the tech community, and considered a leading authority on women’s health. She is a member of the MindBodyGreen Collective and a faculty member for the American Academy of Anti Aging Medicine. Her work has been featured in the New York Post, Forbes, Cosmopolitan, Huffington Post, Bustle, The Guardian, Sports Illustrated, Elle, and ABC News. Read more about me here.