Recently, I got certified as an Open Water Diver and it was really fun until some female divers start talking about menses. Since it does concern me as well, i decide to search the net and found this...

" Heavy flow and menorrhagia may be different than light flow. This may be an increasing concern as the female diving population ages. Most women with very heavy flow probably would skip diving on those days. Issues of hygiene, privacy and convenience. If the flow is very heavy with superimposed anemia, chronic or acute, that would limit potential O2 delivery, and impair circulatory dynamics.

There is some suggestion from aerospace literature and from dry chamber dives for medical hyperbaric therapy that women may be at increased risk for DCS during the first week of their cycle… that is during the menstrual week. This phenomenon has not been studied in open water divers. It might however suggest that due to changes in hormones, electrolytes, vasomotor reactivity and peripheral vasoconstriction, and other unknown variables, women might handle a gas load differently during the menstrual phase of their cycles. One might caution women who dive while menstruating to dive more conservatively… jumping tables, doing shallower dives, and prolonging safety stops."

Source: Divers Alert Network