Female lower back has evolved to accommodate strain of pregnancy

According to a new study by researchers at Harvard and the University of Texas at Austin, women’s lower spines evolved to be more flexible and supportive than men’s to increase comfort and mobility during pregnancy, and to accommodate the special biology of carrying a baby for nine months while standing on two feet.

The study published in the Dec. 13 edition of the journal Nature was led by Katherine Whitcome, a postdoctoral researcher in the Department of Anthropology in Harvard’s Faculty of Arts and Sciences, with Daniel Lieberman, professor of anthropology at Harvard, and Liza Shapiro, associate professor of
anthropology of the University of Texas at Austin.

“Pregnancy presents an enormous challenge for the female body,” says
Whitcome. “The body must change in dramatic ways to accommodate the
baby, and these changes affect a woman’s stability and posture. It turns
out that enhanced curvature and reinforcement of the lower spine are key
to maintaining normal activities during pregnancy.”

It has long been appreciated that giving birth to large-brained infants
has influenced human pelvic shape, but there has been little attention
paid to the major challenge that pregnant bipedal mothers endure when
holding up an enormous fetus and placenta well in front of the hip
joints. The study is the first of its kind to examine the evolutionary
mechanisms that allow women to carry a baby to term, and the way that
women’s bodies compensate for increased weight in the abdomen during
pregnancy.

Walking on two feet, which began early in human evolution, presents a
unique challenge during pregnancy because the center of gravity shifts
far in front of the hips, destabilizing the upper body and impairing
locomotion. This is not the case for animals that walk predominantly on
four legs such as chimpanzees, or even other bipeds.

To accommodate this shifted center of gravity, women’s spines have
evolved to help offset the additional weight in the abdomen during
pregnancy, so that the back muscles are not taxed in counter-balancing
the destabilizing effects of the baby’s weight.

In both women and men, the curvature of the spine in the lower back,
called the lordosis, stabilizes the upper body above the lower body. The
researchers studied 19 pregnant women between the ages of 20 and 40 and
found that when naturally standing, the women lean back, increasing
their lordosis by as much as 60 percent by the end of their term. In
doing so, pregnant women maintain a stable center of gravity above the
hips.

The research also demonstrates, for the first time, that human lumbar
vertebrae differ between males and females in ways that decrease the
shearing forces that the lumbar extension of pregnancy places on the
lower back in pregnant mothers.

“In females, the lordosis is subtly different than that of males,
because the curvature extends across three vertebrae, while the male
lordosis curves across only two vertebrae,” says Whitcome. “Loading
across three vertebrae allows an expectant mother to increase her
lordosis, realigning her center of gravity above her hips and offsetting
the destabilizing weight of the baby.”

In addition to the difference in the number of vertebrae across which
the lordosis spans, the female joints are relatively larger and flare
out further down the spine than those of males, improving the spine’s
strength. All of this contributes to an increased ability to extend the
spine, so that the woman can lean back, realign the body’s center of
gravity, and safely maintain a more stable position. These differences
in the lower back may even reinforce her capability to support and carry
her baby in her arms after the baby has been born.

When human ancestors first became bipedal, they set the human lineage
off on a different evolutionary path from other apes, but in so doing
created special challenges for pregnant mothers. One exciting discovery
is that the ability of human females to better carry a baby to term
while standing on two feet appears to have evolved at least two million
years ago. The researchers studied two hominin fossils that were
approximately two million years old, one of which – presumably a female
– displayed three lordosis vertebrae and one of which – presumably a
male – displayed fewer.

“Early human women lived very strenuous, active lives, and pregnant
females were forced to cope with the discomfort of childbearing while
foraging for food and escaping from predators,” Lieberman says. “This
evolution of the lower back helped early woman to remain more mobile
during pregnancy, which would have been essential to survival, and
appears to have been favored by natural selection.”

The research was funded by the National Science Foundation, the L.S.B.
Leakey Foundation, and the American School of Prehistoric Research.