Menstrual synchrony
Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.
Menstrual cup
While numerous companies all over the world offer this product it was still not well known in around 2010. It may be difficult for companies to make profit from this product as one single menstrual cup can last a girl or woman five years or longer. Most women hear of menstrual cups through the internet or word of mouth, rather than through conventional advertizing on TV for example. As of 2018, menstrual cups are mentioned more and more often alongside tampons and pads in publications about menstrual hygiene management.