Spinal curve keeps pregnant women on balance

Spinal curve keeps pregnant women on balance
NEW YORK (Reuters Health) - During the last weeks ofpregnancy, many women become so large in the middle that theylook as though they might tip over. But through evolutionaryprocesses, nature has devised a way to keep them upright ontheir sometimes swollen feet.
In a new study, U.S. researchers show how the lower spinein females has evolved to support the obstetrical loadexperienced by species that stand upright on two feet, so thatthe center of balance is repositioned over the hips.
"Most previous studies on the evolution of how humanmothers have accommodated pregnancy have focused on the pelvisand the problems of birthing; this is the first study to lookat how mothers cope with the considerable challenges ofstabilization while they are pregnant," senior author Dr.Daniel E. Lieberman, from Harvard University in Cambridge,Massachusetts, told Reuters Health.
The normal curvature of the lower spine helps position thetrunk above the hips in humans, thereby stabilizing the upperbody over the legs, according to the report in the issue ofNature. Pregnancy, however, complicates matters as the weightshifts forward.
In their study, Lieberman's team shows how over time thelower vertebrae in human females have become reinforced toallow the exaggerated curved position of the spine assumed bypregnant women when they are standing upright. This positionallows the trunk's center mass to remain above the hips.
By studying fossils of Australopithecus, the researchersfound that these vertebral changes actually preceded theevolution of Homo sapiens.
"For me, the biggest surprise was that we can see thismaternal adaptation in the fossil record so far back,"Lieberman said. "It makes sense that evolution would havefavored mothers who were better at coping with these demands,but I didn't expect the fossil record would be good enough toyield such evidence."
In future studies, he added, "I think we need to look moreat the costs of this adaptation and how it relates to the backproblems that so many of us regularly experience."
SOURCE: Nature, December 13, 2007.