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Study: Covid-19 Vaccination Slightly Delays Women’s Periods

Vaccinated women in the US study, most of whom received Pfizer or Moderna, had an increase in menstrual cycle length of just under one day, but experienced no change in the number of menses days.

Study: Covid-19 Vaccination Slightly Delays Women’s Periods

KUALA LUMPUR, Jan 9 – New research in the United States has found that Covid-19 vaccination leads to a slight and temporary delay in women’s periods.

The study funded by the US’ National Institutes of Health, which was published in the medical journal Obstetrics & Gynaecology last January 5, showed that vaccinated women experienced an increase in menstrual cycle length of nearly one day compared to unvaccinated women.

Most vaccinated individuals in the study received the Pfizer and Moderna mRNA coronavirus vaccines. On average, the first vaccine dose was associated with a 0.71-day cycle increase in cycle length and the second dose with a 0.91-day increase.

The increase in cycle length – a longer time between bleeding – was not associated with any change in the number of menstrual bleeding days.

“It is reassuring that the study found only a small, temporary menstrual change in women,” said Dr Diana W. Bianchi, director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), in a statement.

Of the 3,959 individuals in the study, 2,403 were vaccinated and 1,556 were unvaccinated. Study authors analysed data from a fertility tracking app called Natural Cycles.

For vaccinated women, researchers looked at data from three consecutive cycles before vaccination and from three more consecutive cycles, including the cycle or cycles in which vaccination took place. For unvaccinated individuals, data was collected for six consecutive cycles.

Study authors said they analysed cohorts with consistent normal menstrual cycle lengths to clearly identify any links between Covid-19 vaccination and cycle and menses length, noting that many women do not fit in this category.

“Our findings are reassuring; we find no population-level clinically meaningful change in menstrual cycle length associated with Covid-19 vaccination,” researchers concluded.

“Individuals receiving two Covid-19 vaccine doses in a single cycle do appear to experience a longer but temporary cycle length change. Coronavirus disease 2019 (Covid-19) vaccination is not associated with changes in menses length.”

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