**Background:** Since the emergence of COVID-19 in December 2019, safe and effective vaccines have been a cornerstone of pandemic control. While common vaccine side effects such as pain, fatigue, and fever are well-documented, menstrual cycle disturbances have been rarely studied despite anecdotal reports. This study aimed to evaluate the association between COVID-19 vaccination and menstrual disturbances among Iranian women, including those receiving inactivated and viral-vector vaccines.
**Methods:** This cross-sectional study used an anonymous online survey conducted via Google Forms from January 1 to April 1, 2022. A convenience sample of Iranian women was recruited through social media platforms including Email, Instagram, Telegram, and WhatsApp. Inclusion criteria were age 16 years and older, voluntary participation, and ability to independently complete the self-administered questionnaire. After excluding 39 participants with gynecological disorders (uterine fibrosis, endometriosis, ovarian cysts) or those taking hormonal medications, 455 participants were included. The questionnaire collected data on age, type of COVID-19 vaccine received (Sinopharm, AstraZeneca, Sputnik, Barkat, Baharat), and self-reported menstrual disturbances after the first, second, and third doses. Outcomes included premature menstruation, latency of menstruation, heavy bleeding, spot bleeding, increased or decreased number of menstrual days, and dysmenorrhea in menstrual or menopausal women. Statistical analysis used descriptive statistics (numbers, percentages) and chi-square tests to evaluate associations between vaccination and side effects. A self-controlled case-series (SCCS) design was employed for the main analysis.
**Key Results:** Approximately two-thirds of participants were over 30 years old. Most participants (>60%) reported no menstrual disorders after vaccination. Menstrual disturbances were reported by 18.4% after the first dose, 20.9% after the second dose, 12.5% after the third dose, and 0.2% after all three doses. Among women of reproductive age, latency of menstruation was the most common disturbance (19.4%), followed by premature menstruation (17.1%). Over 50% of women reported no symptoms after vaccination. Among menopausal women, more than 85% reported no problems, though approximately 7% had a history of endometriosis or ovarian cysts. Relative risks of menstrual disorders by vaccine type were: Baharat 66%, Sinopharm 50.6%, AstraZeneca 40.9%, Sputnik 38%, and Barkat 33% across all doses.
**Clinical Implications:** This study adds to the growing evidence that COVID-19 vaccination can be associated with menstrual disturbances, particularly latency and heavy bleeding. The findings are consistent with UK surveys reporting menstrual disturbances in 20% of vaccinated women and US data showing 42% of women with regular cycles experienced heavier bleeding post-vaccination. The observation that disturbances occurred across different vaccine types (both inactivated and viral-vector) supports the hypothesis that menstrual changes result from immune response to vaccination rather than a specific vaccine component. Biologically plausible mechanisms include immune activation affecting the hypothalamic-pituitary-gonadal axis, disruption of hormone secretion (estrogen, progesterone, FSH, LH), and immune cell-mediated effects on the endometrium. Study limitations include convenience sampling limiting generalizability, online data collection, and cultural/religious factors specific to Iran. The authors emphasize that menstrual disturbances are not a contraindication to vaccination but recommend that information about potential menstrual changes should be included in future clinical trials and public health communications to address vaccine hesitancy.