The idea that women who spend significant time together will eventually have their menstrual cycles align, or “sync up,” is a widely recognized and frequently discussed phenomenon. This concept is known scientifically as menstrual synchrony. Anecdotal confirmation has fostered a belief in a biological connection between women in close proximity. This article examines the historical foundations of this belief and reviews what modern science and statistical analysis reveal about its validity.
The Hypothesis of Menstrual Synchrony
The popularization of menstrual synchrony began with a 1971 study conducted by Martha McClintock. She observed 135 female students living together in a college dormitory. The initial findings suggested that the onset dates of the women’s periods became closer over the course of the academic year, an observation often referred to as the McClintock effect.
The primary biological mechanism proposed to explain this alignment was the influence of pheromones. Pheromones are chemical signals released by one individual that can affect the physiology of others of the same species. The hypothesis suggested that subtle chemical cues transmitted through bodily secretions could regulate the timing of the menstrual cycle in nearby women.
McClintock and her team later attempted to confirm the pheromone mechanism by applying armpit extracts from donor women to recipient women. They reported that extracts collected during the pre-ovulation phase seemed to shorten the cycle, while extracts from the post-ovulation phase appeared to lengthen it. This research suggested that human pheromones could modulate the timing of the menstrual cycle.
However, the foundational 1971 study and subsequent pheromone research faced significant criticism regarding methodology and statistical analysis. Many later studies attempting to replicate the original findings failed to find any evidence of cycle synchrony in women living together. Critiques highlighted issues such as small sample sizes and statistical errors that may have artificially inflated the appearance of synchrony in the initial reports.
Scientific Findings and Statistical Probability
Modern scientific consensus indicates that menstrual synchrony is not a genuine biological phenomenon driven by pheromones or social contact. Large-scale, longitudinal studies using advanced data collection methods, such as period-tracking apps, have provided robust evidence against the idea. A 2017 study conducted by Oxford University in partnership with the app Clue analyzed data from over 1,500 participants and concluded there was no evidence of cycles synchronizing over time.
The perception of synchrony is instead explained by the laws of mathematical probability and the natural variability of the menstrual cycle. A typical cycle ranges from 21 to 35 days, with an average duration of around 29 days. Given these ranges, it is statistically inevitable that the cycles of any two women will overlap at some point purely by chance.
Consider two women with a 28-day cycle and a five-day period. If their cycles start 14 days apart, they are maximally out of sync. If their cycles start just seven days apart, however, their periods will overlap for at least two days. Due to minor, independent fluctuations in individual cycle lengths, start dates will naturally drift closer together and then further apart over time.
The cycles of two women naturally converge and then diverge again over time, creating temporary periods of alignment followed by separation. Observers tend to notice and remember the times when their cycles overlap, which creates a cognitive bias known as “confirmation bias.” They are less likely to recall the far more frequent times when their cycles are not aligned.
This cycle variability, which leads to temporary alignment, is a statistical artifact of two independent cycles progressing naturally. Studies that correct for the probability of chance overlap consistently find that observed synchrony is no greater than what would be expected if the cycles were independent. The conclusion is that women do not synchronize their menstrual cycles.
Factors That Influence Individual Cycle Timing
While external social factors do not cause group synchrony, several internal and environmental factors influence the timing and regularity of an individual’s menstrual cycle. The cycle is controlled by a delicate interplay of hormones regulated by the brain’s hypothalamus and pituitary gland. Disruptions to this system directly impact cycle length.
High levels of psychological or physical stress increase the stress hormone cortisol, which can delay or temporarily stop ovulation, altering the cycle. Significant changes in body weight, either gaining or losing a substantial amount, can also disrupt the hormonal balance necessary for regular cycles.
Intense or excessive physical activity, especially in athletes with very low body fat, can impact hormonal production and lead to irregular cycles. Other lifestyle factors can also introduce temporary irregularities, including poor sleep patterns, sudden changes in diet, or travel across time zones.
Underlying medical conditions, such as thyroid disorders or Polycystic Ovary Syndrome (PCOS), affect the regularity and length of the menstrual cycle. These factors demonstrate that individual cycle timing is sensitive to a woman’s personal physiological state and environment, not the cycles of those around her.