well·6 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
Jan 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: A study in the journal Obstetrics & Gynecology released last week investigated the connection between vaccines and the menstrual cycle. 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. Read Dr. Crawford’s answers to our Qs below.

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

When it comes to a COVID infections and vaccines, 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. A recent study looked at people who received the COVID vaccine and found no clinically significant change in menstrual cycle length (less than a day change in length after receiving the first or second vaccine). 

There was a small subset of people who received both doses within the same cycle, and they were slightly more likely to have a prolonged menstrual cycle length of eight days or more (10.6% vs 4.3%), but this returned to normal after two period cycles. 

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. We still recommend the vaccine, 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?

We define an “irregular period” as 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 Jan. 10 to include new research on the menstrual cycle and the COVID-19 vaccine.

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Skimm'd by Carly Mallenbaum, Becky Murray, and Anthony Rivas

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