Why are some bodily fluids missing from the "Thirty Two Parts of the Body" teaching?
You could probably contemplate on it as blood. Women entered the Sangha somewhat later. So I would presume this meditation was taught to monks before that happened. You might have also noticed that seminal fluid is missing from the list.
Women having to contemplate on that might actually arouse their lust. Maybe it's the same case with men having to contemplate on women's period.