How many women menstruate at the same time in the world?

Some rough figures... 7,000,000,000 people on the planet = 3,500,000,000 women. 70% of those will be either pre-pubescent or post-menopausal. Leaving 1,050,000,000 possibilities. Divide that by the number of days in a year - and you get the average figure of 2,874,743