Pelvis, posture and protrusion: evolution and haemorrhoids


Human locomotion is unique among mammals. We are obligate bipeds who walk erect on two legs. Circus dogs and dancing bears can also walk on two legs, but only awkwardly and for short periods of time. Even our closest biological relatives—chimpanzees and bonobos—are knuckle-walkers, using four limbs for locomotion and assuming an upright posture only briefly and for special activities.

The evolutionary change in our hominid ancestors from quadrupedal locomotion to a bipedal posture required numerous evolutionary changes in the architecture of the human pelvis. The knees moved medially to accommodate changes in the centre of gravity. The gluteal muscles rearranged themselves to provide gait stability in those precarious moments when all of our weight is balanced on one foot as we lift the other one to stride forward. Most importantly, the hip joints were reconfigured, narrowing the mid-pelvis, and the curvature of the spine changed to a springier S-shape. This also changed the pressure vectors in the pelvis. An upright posture moved the viscera directly over the pelvic outlet, requiring new mechanisms of pelvic support. The rise in elevation accompanying upright locomotion also put new strains on the cardiovascular system, requiring more pressure to pump blood upwards but also adding new challenges to the system of venous return as blood ascended from the longer lower extremities up through the reconfigured pelvis.

At the same time, hominids began having larger offspring with larger heads. This combination of a narrower pelvis occasioned by bipedal posture and progressive increases in cranial capacity led to what has been called the ‘human obstetric dilemma’: getting large babies with large brains through a narrow birth canal. The long labours and tortuous obstetric mechanics that often occur also increase the pressures exerted on the gravid pelvis and its surrounding structures.

Pregnancy complicates venous return from the pelvis. There is an enormous increase in circulating vascular volume in pregnant women. Furthermore, the large gravid uterus rests on the great vessels of both the abdomen and pelvis, impeding venous return from the lower regions. The distension of the venous system that results is an ideal setting in which haemorrhoids may blossom in susceptible women.

Poskus et al. have taken on the challenge of exploring the vagaries of haemorrhoid formation in pregnancy. Their prospective study of haemorrhoids and fissures in pregnancy and the puerperium reveals that almost half (43.9%) of pregnancies are complicated by peri-anal disease, the major complaint being haemorrhoids. Not surprisingly, multivariate analysis of their data revealed a common thread: women with conditions that increased intra-abdominal pressure (constipation during pregnancy, straining for more than 20 minutes during delivery, a newborn infant weighing more than 3800 g) or a previous history of peri-anal disease dramatically increased the risk for haemorrhoid formation.

Considering the misery produced by a thrombosed haemorrhoid, perhaps it is time to study whether or not a return to some variant of the high-fibre diet consumed by our hominid ancestors would help to prevent this condition among pregnant women. The exact answers to haemorrhoid prevention remain unclear, but with additional clinical research, we can be confident that the answers will emerge—in the end.

Disclosure of interests

I have no competing interests of relevance to this paper.