Close female friends share everything from clothing to secrets, and recent studies show they might even share menstrual patterns.

While there is no evidence conclusive enough to prove the theory, some scientists believe that women who live in close quarters or who spend a lot of time together develop menstrual synchrony, which means they tend to menstruate around the same time of the month.

Study origin

For many years, this was considered by many women to be nothing more than an old wives' tale.

But in 1968, Martha McClintock, an undergraduate student at Wellesley College, noticed that women in her dorm were menstruating at the same time and began to conduct studies to find out why.

She interviewed many of the women and then gathered and compared data about their menstrual cycles, eventually publishing her findings and becoming the pioneer of menstrual synchrony research, Penn State physician Robert Heinbach said.

Controversy

Today the issue is still being studied, and researchers are debating whether or not menstrual synchrony is an actual phenomenon.

"It is incredibly controversial," Sharon Shriver, assistant professor of biology, said.

"There are good points on both sides [of the argument]."

Studies done within the past couple of years have yielded results supporting McClintock's case that menstrual synchrony does exist.

In one such study, researchers observed the menstrual patterns of a group of women who were brought to live together for the study, Nancy Williams, assistant professor of kinesiology, said.

After several months of living in close quarters, the women's menstrual cycles were compared again.

"[The researchers] found that 30 percent of the women had started to cycle with each other within three months," Williams said.

Occurrence

Studies by Leonard and Aron Weller found that menstrual synchrony occurs between co-workers and between women in some closely knit family units.

Other studies found a greater degree of menstrual synchrony among women who were close friends than among women who did not have close relationships, she said.

Yet another study examined a group of lifeguards over an entire summer and found that by the end of the summer, nearly all of the women were menstruating around the same time, Heinbach said.

However, a study that examined a women's basketball team for an extended period of time found no correlation between the women's menstrual patterns.

They therefore also found no evidence of any occurrence of menstrual synchrony, he said.

Other factors

But Williams pointed out that exercise, dieting and stress can each cause changes in women's menstrual patterns as well.

"[Menstrual synchrony] is not always observed, but I definitely think it is a real phenomenon," she said.

Many women living in dorms and apartments, especially those who share their spaces with female roommates, agree.

"I think it exists, and I just took it to be a fact before I even knew there were so many studies about it," said Beth Golden (senior-music).

But she also pointed out that many women, especially in college, take oral contraceptives, which regulate the menstrual cycle and could be the reason why some women menstruate at the same time.

"It's hard to judge when there are other medications or chemicals in the body," she said.

Causes

The question that many scientists are trying to answer now is: What is it that can cause this possible menstrual synchrony?

Heinbach said most current research experiments deal with pheromones as the main cause of menstrual synchrony.

This is what what McClintock had originally suggested as the cause for the observed phenomenon, but had not confirmed.

Pheromones are chemical messages passed between members of a species, which are odorless but are picked up by the olfactory (sense of smell) nervous system, he said.

"There have been a lot of studies done with animals and insects in the past 20 years," he said.

"Fruit flies, moths, monkeys and dogs in heat have all been proven to respond to pheromones."

Heinbach said pheromone research was recently extended to humans as well.

In one study, a group of women who were in the follicular phase (the first two weeks of the menstrual cycle before ovulation occurs) had pads placed under their arms to pick up their pheromones.

The pheromones were then applied to the upper lips of another group of women, so the "scent" would be directly below their noses.

This second group of women was examined to see if their menstrual cycles would align with those of the women in the first group.

This would signify that pheromones could induce ovulation in women who spend time together.

Results

The experiment showed that the use of the pheromones resulted in an increased amount of Luteinizing Hormone (LH) released in the women's bodies, he said.

He explained that LH is released naturally by the pituitary gland during the follicular phase to trigger the beginning of ovulation.

Thus, in effect, the study showed a correlation between pheromones and ovulation, which could indicate that pheromones are a cause of menstrual synchrony.

Williams said this theory makes sense.

She described another study of a group of women who had spent time living together.

The group was divided into two smaller groups: women who were already cycling together (synchronous) and women who were not (non-synchronous).

Researchers found that, compared to the non-synchronous group, the women in the synchronous group had a higher sensitivity to a certain chemical, she said.

"It shows there is a connection between the olfactory system and the part of the brain that controls reproductive hormones," she said.

"This has already been proven to exist in animals, which rely more on smell than humans do."

She said scientists are also trying to figure out how to measure the occurrence of any menstrual synchrony in women.

"We don't know how powerful it is, but it does exist," she said.