Wellness·5 min read

Why Your Period Could Seem 'Off' After You Get the COVID Vaccine

Let's Talk About...COVID-19 Vaccines and Your Period
Design: theSkimm | Photo: Pexels
January 10, 2022

A few of us at Skimm HQ noticed that our periods seemed a bit ‘off’ after getting the COVID-19 vaccine. We’re not the only ones paying attention to this: Scientists in the US and abroad have also been looking into the connection between vaccines and the menstrual cycle. So we phoned fertility physician Dr. Natalie Crawford and asked her what the data shows, why your period could be impacted, and what to know if you’re getting the COVID-19 vaccine and trying to get pregnant. She helped us answer our Qs below.

What information’s available about whether COVID-19 and the vaccine changes your period? 

First, we know that 25% of women who had a COVID-19 infection reported a change in their period pattern (heavier or lighter, longer or shorter). This change lasted for one to two cycles and then returned to normal.

As for the COVID-19 vaccine and periods, a study published in September 2022 builds on earlier data on this topic. It analyzed period tracker info and found that vaccinated women got their periods an average of 0.7 days late. But people who had two shots during the same menstrual cycle had more of a disruption: almost four days on average added to their cycle. Note: Effects appear to be temporary and have no impact on fertility.

And research published in July 2022 found that survey participants who menstruated regularly were about as likely to experience heavier periods after vaccination (42%) as they were to experience no changes to their bleeding at all (42%). It also found that the majority of people who didn’t menstruate regularly and were postmenopausal or on long-acting reversible contraceptives (see: IUDs, implants) experienced heavier bleeding. But the researchers cautioned that their findings aren’t causal and don’t represent the general population. And that there’s no reason to feel alarmed.

What might cause the menstrual cycle to shift?

Because the endometrium (aka the lining of the uterus) is an immune-responsive tissue, it’s possible that when the body has a large immune response (i.e. making COVID-19 antibodies either from a COVID-19 infection or the COVID-19 vaccine), it may make the endometrium unstable and cause changes in bleeding.

Does the COVID-19 vaccine impact a woman's ovulation window and ability (or timing) to conceive? 

Studies have shown that in women who’ve had either the COVID-19 infection or received the COVID-19 vaccine, there’s no change to ovarian function (ovulation and hormone production) or ovarian reserve (how many eggs you have). Hypothetically, a change in your bleeding pattern may make it harder to conceive in that particular cycle. However, the majority of people are not experiencing abnormal bleeding after the COVID-19 vaccine. We also have consistently good data showing that a COVID-19 infection during pregnancy can cause severe maternal and neonatal complications. So getting vaccinated before — or when — you’re pregnant is important. All professional societies who care for pregnant people or those trying to conceive recommend vaccination. The CDC now recommends vaccination after a large study (over 35,000 people) showed no increase in adverse events during pregnancy. (You can read more about what experts say about pregnancy and the vaccine.)

What do women on fertility medication and/or planning an IUI procedure or IVF cycle need to know about how the COVID-19 vaccine may impact them?

If you’re undergoing fertility treatment, please talk to your doctor. The vaccine is still recommended, but not within three days of a procedure that could result in implantation. Implantation requires a proper immune response. It’s common to cancel procedures like IUI or an embryo transfer if you’re sick with any illness — even before the pandemic. The reason: Your immune system may have a hard time responding appropriately for implantation while it’s busy creating antibodies to your illness. Thing to know: Studies have shown there’s no change in IVF outcomes after COVID-19 vaccination.

Does stress play a part in changing the timing or heaviness of a period?

Stress may play a role. Typically the body's response to a stressful situation is to shut off the brain's production of the hormones that allow you to ovulate (FSH and LH). This is called hypothalamic amenorrhea. When this happens, we see absence of periods for a prolonged period of time. This may explain why there are some reports of amenorrhea after a severe COVID-19 infection, and it’s unlikely after the COVID-19 vaccine because any reaction is very temporary and short-lived.

When should someone be concerned about their irregular period?

An “irregular period” is one that does not come at a regular and predictable interval. If you have a change in your period pattern for more than two cycles, you should see a doctor for an evaluation. Note: You are more likely to have period changes from a COVID infection than from the vaccine. Irregular periods can also be caused by thyroid disease, pituitary dysfunction, PCOS, hypothalamic amenorrhea, chronic illness, and many other causes that require medical attention. Remember that your period is a "vital sign" and gives you information about your body. 


The endometrium (the lining of the uterus) is an immune-responsive tissue. So it’s possible that when your body has a large immune response (i.e. making COVID-19 antibodies from the COVID-19 vaccine), that it may make the endometrium unstable and cause temporary bleeding changes. Contact your doctor if your period doesn’t return to normal after two cycles.

Updated on Sept. 27 to include new research on the menstrual cycle and the COVID-19 vaccine.

This content is for informational and educational purposes only. It does not constitute a medical opinion, medical advice, or diagnosis or treatment of any particular condition. 

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