According to a Swedish study published in the BMJ that analyzed nearly 3 million women, no substantial evidence was found that predicted a causal association between SARS-CoV-2 vaccination and healthcare contacts related to menstrual or bleeding disorders.

Many women have reported changes in their menstrual cycles after receiving a COVID-19 vaccination, in terms of the number of days they bleed and the heaviness of the flow. As a result, the study enrolled 2,946,448 women aged 12–74 who were vaccinated against COVID. Over two-time windows, i.e., 1–7 days, considered the control period, and 8–90 days, the risks were evaluated for Pfizer-BioNTech, Moderna, or Oxford-AstraZeneca vaccination in association with dosage status.

During the study period, more than 2.5 million (88%) women had received at least one COVID vaccination, and more than 1.6 million (64%) of vaccinated women had received three doses. The findings of the study showed an increased risk of postmenopausal bleeding of 23–33% for the Pfizer-BioNTech jab after 8–90 days and after three doses of the Moderna shot.

In premenopausal women, there was a modest link between menstruation disruption or bleeding and any dosage of vaccinations, i.e., 13% or 8% after 1–7 days and 6% or 1% after 8–90 days, respectively. The highest risks for bleeding in postmenopausal women were observed after the third dosage in the 1–7 day risk window (28%), and in the 8–90 day risk window (25%). 

(Source: https://medicalxpress.com/news/2023-05-menstrual-covid-vaccination.html )