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.
Extended cycle combined hormonal contraceptive
With all extended-cycle COCPs, breakthrough bleeding is the most common side effect, although it tends to decrease over time. In a 12-month study of a continuous COCP regimen, 59% of women experienced no bleeding in months six through twelve and 79% of women experienced no bleeding in month twelve. Extended or continuous use of COCPs or other combined hormonal contraceptives carries the same risk of side effects and medical risks as traditional COCP use. 
One of the early extended-cycle COCPs, Seasonale, was marketed with the campaign, ""Fewer periods. More possibilities."" In December 2004, Barr Pharmaceuticals was warned by the FDA concerning these television advertisements. As the warning stated, ""By omitting and minimizing the risks associated with Seasonale, the TV ad misleadingly suggests that Seasonale is safer than has been demonstrated by substantial evidence or substantial clinical experience."" Although clinical studies had proven Seasonale to be effective in preventing pregnancy, the FDA felt the commercial advertisements omitted the common side effects of irregular vaginal bleeding or spotting.