Directionality of menstrual flow: cervical os diameter as a determinant of retrograde menstruation

OBJECTIVE: To develop a mathematical model to examine the factors that control the directionality of menstrual flow. DESIGN: The model consisted of a rigid cavity, filled with a viscous liquid, with three outflow ports: a set of paired outflow ports (fallopian tube ostia) and an additional single outflow port (cervical os). The Darcy-Weisbach equation was used to calculate flow rates through the ports. RESULTS: At cervical os diameters of less than 0.5 mm, more than 50% of flow was through the fallopian tubes over a wide range of values for cervical length and fallopian tube ostia diameter and length. The diameter of the fallopian tube ostia was also an important determinant of the directionality of flow. The lengths of the cervical os and fallopian tube ostia were of secondary importance in determining directionality of menstrual flow. CONCLUSION: If retrograde menstruation is a risk factor for the development of endometriosis, menstruating women with cervical os diameters less than 2.0 mm may be at increased risk for developing endometriosis.