One of the first studies to track whether COVID-19 vaccination might affect women’s menstrual periods found a small temporary change.

The study published on Wednesday tracked the six menstrual cycles of nearly 4,000 American women. On average, the next period after the injection was a day later than usual. However, the number of days of menstrual bleeding after the COVID-19 vaccine has not changed.

“This is very reassuring,” said Dr. Alison Edelman of Oregon Health and Science University, who led the study and said that it is important to tell women what will happen.

Some women report irregular menstruation or other menstrual changes after the injection. The National Institutes of Health is funding research to check if there are any links.

Edelman’s team analyzed data from a birth control app called Natural Cycles, which has been approved by the U.S. Food and Drug Administration to allow women to track their menstrual cycles and determine when they are most likely to become pregnant.

The menstrual cycle is calculated from the first day of one cycle to the first day of the next cycle. Slight changes every month are normal, and stress, diet, and even exercise can stimulate temporary changes.

Edelman said the study included women with the “most normal” cycle length, which averaged between 24 and 38 days. The researchers followed the vaccinated women during the three cycles before and after the injection, including the month they were vaccinated-and compared them with unvaccinated women. The app prompts women to enter vaccine information.

Among the 358 women who received both vaccines in the same menstrual cycle, the change in the length of the next cycle was slightly larger, with an average of two days. Researchers reported in the journal Obstetrics & Gynecology that about 10% of them had changes for 8 days or more, but then returned to normal ranges.

Edelman said that one theory is that when the immune system accelerates at a certain time in the cycle, “our biological clock or something that controls the menstrual cycle may hiccup.”

She plans to conduct additional studies to determine whether there is a change in the weight of menstrual bleeding, or whether women with irregular menstruation react differently.

Dr. Christopher Zahn of the American College of Obstetricians and Gynecologists said in a statement that these findings provide “important new evidence, emphasizing that any impact of the COVID vaccine on menstruation is minimal and temporary.”