The World Boxing Council recently issued a statement to the effect that it would no longer sanction any fight between female boxers that is scheduled to go longer than 10 rounds or any bout with rounds lasting longer than two minutes. Part of the reason they have made this decision is because ladies be gettin' periods.
Here's a portion of the statement (emphasis mine):
Several facts were reported during the convention which are of total concern regarding any changes of the 10 x 2 female boxing matches :
-Women are physically inclined to have concussions , much more than men.
-A female fighter used to fight 10 x 2 which is 20 minutes, would need tremendous effort to adjust to 12 x 3 which is 36 minutes.
-Menstrual cycle has tremendous impact on the body of a woman, including 12 hormones which act in the body system, creating radical changes in several areas.
-Women's endurance has been proven to be less than men. Marathon running time is much different between male and female.
That bit about menstruation sounds like something you'd read an 18th century medical journal written by an ether-huffing quack. "Present within the female body are a variety of ill humors, which can work to sap both mental and physical constitution when left unchecked. Bouts of hysteria and crippling fatigue are not uncommon."