Unlocking Menstrual Mysteries: Why do women sync up?
Dear Jo & Ross: Here’s a question for you: why do women who live together eventually “sync up” and have their periods all at the same time?-JTT
Great question; thanks for writing in! Most of us have either experienced this phenomenon-if we are menstruating women-or have heard about it from our fertile female friends. Although many people believe that women who live or spend a lot of time together will have periods that adjust slightly to eventually sync up, the science to prove this actually happens is lacking.
The 28-day model is often used as the example when explaining the monthly menstrual cycle of women. However, cycles vary in length from around 24 to 38 days; they can also be shorter or longer. Cycles vary from women to women but can also differ for an individual woman from month to month. It is because of this that women who live together who did not start out experiencing periods at the same time can, over time.
A menstrual cycle can be divided into two parts: pre-ovulation (when an egg is released) and post-ovulation. It is the time frame pre-ovulation that varies, while post-ovulation usually lasts a consistent 14 days. Factors that affect the length prior to ovulation include stress, weight change, medication, illness or hormone changes. Shorter cycles (24 days) indicate that there were fewer days before ovulation than during a longer cycle (38 days).
There are a couple of theories that attempt to explain why women’s cycles change and become in sync. One theory suggests that women who spend time together experience similar stressors that delay ovulation and therefore alter their menstrual cycles so they eventually all have their period around the same time. The problem with this theory is that if four women had different periods and all experienced a delay in ovulation, they should still experience periods at staggering intervals.
The McClintock Effect
Another theory is based on research done in 1971 by psychologist Martha McClintock, published in Nature. McClintock’s study showed that the pheromones released by women are detected by other women in close proximity, which shortens or lengthens their menstrual cycles. This is also known as the McClintock Effect and is the mostly widely known study of this aspect of menstruation.
Others believe that the normal variation from women to women each month simply causes women over time, to coincidentally get their periods at the same time. For example, a woman with a 27-day cycle might originally have her period at a different time than her roommate who has a 29-day cycle. Over time, the first women will ovulate (if her cycle is consistent each and every month, which is rare) two days earlier than her roommate. The next month, she would ovulate four days earlier than her roommate and eventually, these two women will get their period at the same time.
Nothing to Prove
In fact, most studies are based on McClintock’s original research model, and scientists have found errors in her research design. When these errors are corrected, no significant levels of menstrual synchrony occur.
A research study done in 1998 by McClintock confirmed her earlier findings of synchrony. When underarm odors of women who were ovulating were put under the noses of women in the study it delayed their ovulation and lengthened their cycles. However, no studies have shown that women with the same cycle length have synchronized successfully or that if their cycles started out at different lengths that by being in close proximity their cycles have changed in ways not normally expected.
Theory Behind the Theories
Evolutionary biology provides one theory as to why women’s cycles would sync up. In ancient societies, many women would have sex with one man. Pheromones released from women unconsciously increased sexual desire in men. If a man felt horny with multiple women around, it would be very difficult to pick the woman who was actually ovulating to have sex with her. But if all the women were ovulating at around the same time that a man desired to have sex, the chances of pregnancy and reproduction would increase when mating occurred.
Many women we’ve talked to stick by the claim that when they have lived with other women, their cycles not only synced up, but their periods also started within a day or two of each other every month thereafter. Although science has not yet proven this, there may be more to understand about women, menstruation, fertility and sexuality.
Thanks for the great question, and keep doin’ it well!
Sex 411: Must Be The Moon
• Lunaception- the idea that menstrual cycles synchronize with the moon, is not currently accepted by the scientific community.
Jo & Ross wish you a wonderful New Year! Send them your questions to: firstname.lastname@example.org.