STEVE INSKEEP, HOST:
Some people who received the COVID vaccine have reported changes to their menstrual cycle.
NPR's Geoff Brumfiel says scientists are asking why.
GEOFF BRUMFIEL, BYLINE: Clinical trials for the COVID-19 vaccines looked for side effects like sore arms, but one thing they didn't check for was changes to the menstrual cycle.
ALISON EDELEMAN: The menstrual cycle is like the, you know, stepsister that gets ignored.
BRUMFIEL: Allison Edelman is an OB-GYN at Oregon Health & Science University in Portland.
EDELEMAN: It's considered unimportant in the grand scheme of things, but it actually is really important to people day to day.
BRUMFIEL: When it came to getting vaccinated against COVID, many people did notice changes to their menstrual cycle. And because those changes were never studied in the clinical trials, they sometimes became the basis for conspiracy theories about COVID vaccines. Now, it's known that vaccination is safe for those who are pregnant. But Edelman still wanted to check on menstruation because it affects so many people every month. So she took data from a popular app people use to track their menstrual cycles, and sure enough, she found a minor shift.
EDELEMAN: What we found is that in individuals who were vaccinated versus individuals who have not, we see a less-than-one-day change in their menstrual cycle length with vaccination.
BRUMFIEL: In other words, people experienced, on average, a slightly longer time between bleeding around their first and second dose. The results appear in the journal Obstetrics & Gynecology. And Edelman is now following up on other changes, like heavier flows. She stresses there's no evidence any of this should be worrying.
EDELEMAN: We have not seen anything that's concerning in regard to fertility or pregnancy in terms of vaccination. And in fact, the risk of COVID-19 disease in pregnant women is incredibly serious.
BRUMFIEL: She's seen pregnant women end up in intensive care because they're not vaccinated. Still, Edelman thinks that changes to menstruation probably should be added as a possible minor side effect to the vaccines, along with headaches and sore arms.
Geoff Brumfiel, NPR News.
NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.