For those following this topic, here is a response that I received to the
question on a fertility awareness list.  I received several responses, each a
lesson in pheromones, but I thought this response the most interesting.

That is due to pheromones. Pheromones are hormones present in sweat which
can travel through the air and affect other humans. In other animals,
pheromones often tell a male of the species when a female is in heat. Human
females who are regularly exposed to the pheromones of another female will
often synchronize their periods. In fact, they've done a study where
researchers took sweat samples from females in one group by sticking a
cotton pad under their arms to collect sweat, then they would have women in
another group who never met the women in the first group inhale the scent
from the cotton pads (gross, I know). These women's periods became
synchronized even though they never met! Just from being exposed to their
pheromones in sweat.  The human body is a wonder.

Robin

Nita