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Menstrual flow and timing changes following COVID-19 infection or vaccination

Acute COVID-19 Illness and Menstrual Cycles

During hospitalization for acute illness with COVID-19 infection, younger women in Wuhan, China’s outbreak experienced changes in their menstrual cycles1.  The most common change was to have menstrual cycles that were 33 or more days apart (that occurred for 42% of the 237 women studied)1. These longer cycles were significantly more common in the sicker women who required ventilators or intensive care 1.  Any acute illness can suppress women’s reproduction, as we have known for eons. For example, menopausal women in a 1977 study who were hospitalized with deathly illness had very low levels of luteinizing hormone (LH), so low that they were similar to LH levels in menstruating women (typically 5-15). As women were treated and recovered, LH levels increased toward the normal high menopausal levels (in the 50-150 range)2.  Similar, lower-than-expected LH levels (although the authors did not comment on it) were visible in the very sick women with COVID-19 1.

Illnesses, weight loss and emotional/social/psychological stress are well known to alter women’s menstrual cycles. The brain (hypothalamus) has a protective role; it makes adaptations to protect women from pregnancy when under duress. Brain signals cause lower gonadotrophins (the two hormones called LH and follicle stimulating hormone [FSH] that are part of a complex coordinating system for women’s reproduction. The visible effect of that adaptative suppression of reproduction is longer-than-usual menstrual cycle lengths (4-5 weeks apart) and perhaps lighter flow related to lower estrogen levels. But the first and more subtle effects of stressors—disturbances of ovulation (egg release and thus lower progesterone levels)—are not visible. Silent ovulatory disturbances are more common but are not usually recognized since they occur within clinically normal menstrual cycles3 4. (Here’s a link to the International Olympic Committee’s description of such adaptations to the physical, nutritional and emotional stressors experienced by competitive women and men athletes, a phenomenon that they call “Relative Energy Deficiency-Sport”

COVID-19 Vaccine and Menstrual Cycle Changes

As vaccinations against the SARS-CoV2 virus are rolled out world-wide and premenopausal women are being vaccinated, younger women are now increasingly reporting menstrual period changes after receiving the vaccine. There are no systematic data on this currently, but most commonly reported are unexpected (early) spotting or flow, heavy flow with blood clots, or delayed, skipped or lighter menstruation.  It is puzzling to understand how this might occur.

I believe what women say. I just don’t understand quite how this would be triggered in normal cycles or if it is causally related to having the vaccine. My immediate response when this was first reported a couple of weeks ago was to assume that women noticed because were paying more attention and therefore attributed the observed change to having the “jab”. However, there is striking similarity between what is now being reported and a recently published study (from 2,257 people in an internet survey) following police use of “tear gas” in breaking up Black Lives Matter demonstrations in Portland, Oregon in 2020 ( The women reported similar acute changes in menstruation as are now being reported following vaccination. The majority of women (over 1000) reported changes in cycles or flow 2-4 days after tear gas exposure. Women exposed to tear gas also reported increased cramps. Some women reported breast tenderness (that can indicate higher estradiol levels) about five days after the tear-gas exposure.

My understanding of menstrual cycles that are normally ovulatory (with egg release, and enough progesterone to balance the actions of that cycle’s estrogen5) is that they are unlikely to be acutely changed. However, silent ovulatory disturbances are very common; they occurred in 37% of a single sampled normal-length cycle in a population-based study in over 3,000 women aged 20-49 in Norway6. Ovulatory disturbances are also frequent and normal in the first years of menstruation in adolescence7 8, and in perimenopause9 when heavy flow and increased cramps are also common10. Therefore, I guess we will eventually know that these flow-related changes occur more commonly during COVID-19 illness or following vaccination in those ages 12-19 and in their 40s and in cycles with subclinical ovulatory disturbances in women in their 20s and 30s.  

There are several ways in which COVID-vaccine (and potentially police tear gas exposure) might change menstrual cycles in adolescent-to-perimenopausal menstruating women. One possibility is the acute stress (increased cortisol and epinephrine/norepinephrine stress hormone release) stimulates adrenal glands to also make hormones (like DHEA) that can be converted into estrogens. That surge could trigger an already thickened endometrium (lining of the uterus) to cause flow. There are also known to be very acute stress-related increases in clotting first documented by Cannon of “fight or flight” fame in 1914 11. That might also destabilize the endometrium as the clot dissolves. The sore breasts and heavy flow are likely accounted for by higher estrogen levels. It is possible that immune changes are also related but these would seem to me to take more time to develop.

What is reassuring is that all of these reproductive changes are reversible, whether following vaccines or during the acute SARS-CoV2 illness, according to the original Wuhan study1. Heavy flow requires attention; it seems to occur most commonly (judging both by media reports and those women who have written to CeMCOR) in adolescent and perimenopausal women. If heavy flow occurs following vaccination women can easily and cheaply help themselves by taking 200 mg of ibuprofen (a normal tablet size, widely available without a doctor’s prescription) with every meal on every heavy flow day. Ibuprofen decreases heavy flow by 20-50%. More information about heavy flow is available here

The early/unexpected or heavy flow following vaccination may be a helpful clue to the possibility that a woman is experiencing disturbed ovulation (which is common within regular cycles especially under the multiple stresses of this pandemic). Ovulation can be restored to normal by improved nutrition and sleep, more social/emotional support, increased self-knowledge that can be gained, for example, by tracking menstrual cycle experiences, and ovulation, or, if necessary, by several months of cyclic progesterone therapy

- Jerilynn C. Prior BA, MD, FRCPC

Reference List

1. Li K, Chen G, Hou H, et al. Analysis of sex hormones and menstruation in COVID-19 women of child-bearing age. Reprod Biomed Online 2021;42(1):260-67. doi: 10.1016/j.rbmo.2020.09.020 [published Online First: 2020/12/09]

2. Warren MP, Siris ES, Petrovich C. The influence of severe illness on gonadotropin secretion in the postmenopausal female. Journal of Clinical Endocrinology and Metabolism 1977;45:99-104.

3. Prior JC. The Menstrual Cycle. Its biology in the context of silent ovulatory disturbances. In: Ussher JM, Chrisler J, Perz J, eds. Routledge international handbook of women's sexual and reproductive health2020:39-54.

4. Schliep KC, Mumford SL, Vladutiu CJ, et al. Perceived Stress, Reproductive Hormones, and Ovulatory Function A Prospective Cohort Study. Epidemiology 2015;26:177-84.

5. Prior JC. Women’s Reproductive System as Balanced Estradiol and Progesterone Actions—a revolutionary, paradigm-shifting concept in women’s health. Drug Discovery Today: Disease Models 2020;32:31-40. doi:

6. Prior JC, Naess M, Langhammer A, et al. Ovulation Prevalence in Women with Spontaneous Normal-Length Menstrual Cycles - A Population-Based Cohort from HUNT3, Norway. PLOS One 2015;10(8):e0134473.

7. Metcalf MG, Skidmore DS, Lowry GF, et al. Incidence of ovulation in the years after the menarche. J Endocrinol 1983;97(2):213-19.

8. Vollman RF. The menstrual cycle. In: Friedman EA, ed. Major Problems in Obstetrics and Gynecology, Vol 7. Toronto: W.B. Saunders Company 1977:11-193.

9. Metcalf MG. Incidence of ovulatory cycles in women approaching the menopause. Journal of Biosocial Science 1979;11:39-48.

10. Prior JC. Clearing confusion about perimenopause. B C Med J 2005;47(10):534-38.

11. Cannon WB, Mendenhall WL. Factors affecting the coagulation time of blood—the hastening of coagulation in pain and emotional excitement. American J. of Physiology 1914; 33: 251-261.

Updated Date: 
May 18, 2021

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