Thank you for reaching out to us.

There are different approaches to understanding the seeming disconnect between our sources and women's experiences of hargashah. The main explanations are: (1) that our fundamental halachic understanding of the role of hargashah is mistaken, (2) that our definitions of hargashah are lacking, (3) that there has been a physiological change, or (4) that there has been a change in perception. (See Rabbanit Henkin's discussion of hargashah in Nishmat Habayit, here, for a summary of the core issues.)

The first explanation is difficult to suggest, since the halachic tradition of considering hargashah a necessary condition for a woman to be niddah d'orayta is by now deeply entrenched. The second explanation, which might allow for a wider definition of hargashah (in the way the definition was expanded to include the sensation of liquid flowing), would be extremely challenging (and perhaps unwise) to apply now in an era where couples seem to find it increasingly difficult to keep niddah at all. The third explanation, as you note, does not seem to be medically attested.

This leaves us primarily with the fourth explanation, that women's experiences of the physical sensations of niddah have changed. Given that women have many more menstrual cycles now over a lifetime than historically was the case (due to improved nutrition, earlier age at menarche, reduced time pregnant and nursing, and longer life spans), that modern menstrual hygiene products are substantially more effective than past methods, and that modern Western societies largely educate women to ignore the physical manifestations of menstruation (as opposed to stigmas or isolation), it seems to us quite likely that physical perceptions of niddah have evolved.

Research in this area is limited, tending to focus on taboo, hygiene, and pain. At the same time, Israel's Sherman Method and France's Flux Instinctif Libre (in English, Free Instinctive Flow) are modern movements devoted to enhancing women's awareness and control of their menstrual cycles. Though these methods have not been sufficiently studied and do not claim to create a full level of menstrual continence, their adherents report substantially more sensitivity to the physical signals of actual blood flow. 

In practice, we follow the explanation that there has been a change in women's experience of hargashah (likely a change in perception rather than physiology) to allow for broad leniency with staining. On the same logic, we treat blood seen to exit the body or large quantities of blood (what we typically term a flow) as bleeding that definitely makes a woman niddah, with the presumption that she had a hargashah of which she was not aware.

Please let us know if we can be of further assistance.