I am pleased that my commentary1 has excited interest in the possibility that some habits of life in developed countries might generate abnormal pressure gradients between the uterus and fallopian tubes and hence favour retrograde menstruation.
At least one other cause of retrograde menstruation can be envisaged. During the first three days of the menstrual cycle uterine contractions normally start at the fundus and move towards isthmus and cervix. But pinching or palpitation of the cervix has been observed sometimes to reverse the pattern of uterine contraction in menstruating women2. So another pressure-related hypothesis for endometriosis could be that retrograde uterine contractions might be stimulated by sexual intercourse. Could some of the puzzling geographical distribution of endometriosis be culturally determined? (Intercourse during menstruation is forbidden by some religions, e.g. Ezekiel 18, v. 6.)
A comprehensive epidemiological study investigating all these hypotheses would be formidably difficult, but not impossible. As I pointed out, indirect studies of retrograde menstruation by hystero-salpingo-scintigraphy3 might be practicable if the technique could be quantified. Direct physiological measurements of some of the relevant pressures by manometers connected to fine catheters have already been made in menstruating women2 and could be extended if co-operative volunteers could be found.