Retrograde menstruation happens when menstrual blood flows backward through the fallopian tubes and into the pelvic cavity instead of exiting through the cervix and vagina. This occurs to some degree in most people who menstruate, but certain anatomical and physiological factors can increase the volume of backward flow, raising the risk of complications like endometriosis.
How Menstrual Blood Flows Backward
During a normal period, the muscular wall of the uterus contracts to shed its lining, pushing blood and tissue downward through the cervix. But the fallopian tubes are open at both ends, connecting the uterine cavity to the pelvic space. When pressure inside the uterus builds during these contractions, some menstrual fluid can be pushed upward through the tubes rather than downward. The fluid then spills into the pelvic cavity, where it contacts the surfaces of the ovaries, bowel, and the thin membrane lining the pelvis.
This isn’t rare or abnormal on its own. Studies examining peritoneal fluid (the fluid that sits in the pelvic cavity) consistently find menstrual debris in the vast majority of menstruating people, suggesting that some degree of retrograde flow is nearly universal. In most cases, the immune system clears the displaced tissue without any problems.
Anatomical Factors That Increase Backward Flow
While some retrograde flow is common, certain structural issues in the uterus or cervix can significantly increase its volume.
Cervical stenosis is one of the clearest examples. When the cervical canal narrows or stiffens, it partially blocks the normal downward exit for menstrual blood. With less fluid able to pass through the cervix, pressure builds inside the uterus, and more blood is forced upward through the fallopian tubes. Cervical stenosis can result from surgical procedures on the cervix, radiation therapy, infections, or natural anatomical variation.
Abnormalities in the uterine wall also play a role. The middle muscular layer of the uterus, called the myometrium, is responsible for the contractions that expel menstrual blood. When this layer has structural irregularities, contractions may become disorganized, pushing blood in the wrong direction. Adenomyosis, a condition where uterine lining tissue grows into the muscular wall itself, is one common cause of these irregularities. People with adenomyosis often have heavier periods and more intense cramping, both of which reflect the kind of abnormal uterine activity linked to increased retrograde flow.
Uterine Contractions and Pressure
The uterus doesn’t just squeeze once during a period. It undergoes rhythmic, wave-like contractions throughout menstruation, similar to the peristalsis that moves food through your digestive tract. These contractions normally travel from the top of the uterus downward, directing blood toward the cervix.
In some people, these contractions become hyperactive or move in the wrong direction, a pattern researchers call myometrial hyperperistalsis. When contractions are too forceful or discoordinated, they create excess mechanical stress on the uterine wall. This can detach fragments of the uterine lining more aggressively and push them upward through the fallopian tubes with greater force. Heavy menstrual bleeding compounds the problem: a larger volume of blood combined with increased intrauterine pressure can exceed the natural resistance of the junction between the uterus and the fallopian tubes, opening the path for more retrograde flow.
Why It Matters for Endometriosis
Retrograde menstruation is the leading explanation for how endometriosis develops. The theory, first proposed by surgeon John Sampson in the early twentieth century, suggests that menstrual tissue carried into the pelvic cavity by retrograde flow can implant on pelvic surfaces, respond to hormonal cycles, and grow into endometriotic lesions. This mechanism likely accounts for the majority of pelvic endometriosis cases, particularly lesions found on the ovaries and the peritoneal lining.
But retrograde flow alone doesn’t fully explain the disease. Since most menstruating people experience some backward flow yet only about 10% develop endometriosis, something else determines who is affected. Differences in immune surveillance, the inflammatory environment of the pelvis, and the ability of displaced endometrial cells to establish a blood supply all appear to influence whether retrograde tissue gets cleared or takes hold.
The theory also has notable blind spots. Endometriosis occasionally appears in locations far from the pelvis, including the lungs, diaphragm, and even the brain. These cases can’t be explained by menstrual tissue traveling through the fallopian tubes. Rare cases of endometrial-like tissue in individuals assigned male at birth also point to alternative pathways, likely involving cells left over from early embryonic development rather than menstrual backflow. For pelvic disease, though, retrograde menstruation remains the most well-supported explanation.
Factors That May Increase Your Risk
Because retrograde menstruation is so common, the more relevant question is what makes some people experience enough of it, or the right conditions alongside it, to develop problems. Several patterns emerge from the research:
- Heavy or prolonged periods: More menstrual volume means more potential retrograde flow and more tissue available to implant in the pelvic cavity.
- Short menstrual cycles: Cycling more frequently means more total episodes of retrograde flow over time.
- Cervical narrowing or obstruction: Anything that restricts the normal downward exit for menstrual blood increases the pressure driving fluid backward.
- Adenomyosis: The disorganized contractions and heavier bleeding associated with this condition create conditions for greater retrograde flow.
- Structural uterine abnormalities: Variations in uterine shape or the muscular wall can alter how contractions direct menstrual blood.
These factors don’t guarantee endometriosis or any other complication. They simply increase the volume or frequency of retrograde flow, which is one piece of a more complex puzzle involving immune function, genetics, and the local pelvic environment.