FRIDAY, Dec. 14 (HealthDay News) -- Pregnant women cope with the significant weight gain associated with pregnancy by adjusting the degree of lumbar lordosis, and the additional shearing forces this generates are mitigated by incorporating three vertebrae in the process of dorsal wedging, rather than the two that are used by men, according to a letter published in the Dec. 13 issue of Nature.
Katherine K. Whitcome, Ph.D., of Harvard University in Cambridge, Mass., and colleagues conducted a study of 19 pregnant women to see how lordosis, the posterior concavity of wedged lumbar vertebrae that positions the trunk's center of mass above the hips and thereby stabilizes the upper body over the lower limbs, changed over the course of pregnancy.
As the fetal load increases, women adjust the degree of lordosis, extending their lower back by up to 28 degrees to shift the trunk's center of mass. The consequences of this compensation are greater spinal shearing, which increases the risk of forward displacement of the lumbar vertebrae and causes higher incidence of lower back pain. However, this shearing is reduced by the fact that three lumbar vertebrae are involved in lordosis, whereas in men only two vertebrae are involved.
"The evidence for lumbar sexual dimorphism in humans which improves maternal performance in posture and locomotion suggests that the distinctive hominin lumbar curve has been subject to strong selection pressures," the authors write.
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