The shape and density of women’s spines put them at greater risk for developing scoliosis and osteoporosis. This physical difference in spinal structure between the sexes is unique to humans, but it’s purpose has long puzzled scientists. A recent study, however, has provided evidence to suggest these differences are present at birth, and that they occur because a woman’s spine serves an integral role in helping women adapt to the physical challenges of pregnancy.

For the study, now published in the August issue of the Journal of Pediatrics, a team of researchers at Children’s Hospital Los Angeles used magnetic resonance imaging (MRI) to study the dimensions of 70 full-term newborns (35 boys and 35 girls). According to the press release, newborns’ body lengths, heads, waist circumferences, and weight did not differ greatly between the sexes. However, the strength of the female newborns’ vertebrae, as measured by their vertebral cross-sectional dimensions, consistently measured about 10.6 percent less than that of the male newborns. This finding is important because not only does it mark the first time that differences in spinal structure were found in children so young, but it also hints at the trait’s greater purpose.

"Although we've known that girls had smaller vertebrae than boys, we did not know how early this difference first occurred," senior author Vicente Gilsanz explained in a statement. "Our study indicates that the distinction between sexes is already present at birth and provides new evidence that this difference begins during prenatal development of the axial skeleton."

The team proposes that the differences in newborn spinal strength may be caused by a combination of complex interactions, including sex steroids, growth hormones, and insulin-like growth factors. Unfortunately, having a smaller spine also means that women won’t accumulate the same amount of bone density as men. This decreased bone density weakens the spine and makes women two to four times more likely to sustain a spinal fracture in their lifetime. The smaller size of the spine also gives women more spinal flexibility, putting them at an increased risk of developing scoliosis, or curvature of the spine, later in life.

Despite these health consequences, the unique dimensions of women’s spines allow them to shift their weight during pregnancy. This is an indispensable adaptation, as it allows them to carry a baby for nine months without causing grave bodily harm. Without the ability to shift the spine, the weight of the fetus would cause immense pressure on a woman's hips, making her largely immobile and putting her at risk of serious injury.

A study released earlier this year suggested that the biological advantage of a smaller, more flexible spine may be partly behind men’s preference for women with larger butts. Along with a lower mass and increased flexibility, the female spine also has a curvature of about 45 degrees. Researchers hypothesized that this unique curve helps women shift their weight during pregnancy, and men developed an attraction to these women because the curve increased their chances of having offspring that would live long enough to pass along their DNA.

“Men may be directing their attention to the butt and obtaining information about women's spines, even if they are unaware that that is what their minds are doing,” lead researcher Dr. David Lewis said, according to the Daily Mail.

Source: Ponratana S, Aggabao PC, Dharmavaram NL, et al. Sexual Dimorphism in Newborn Vertebrae and Its Potential Implications. The Journal Of Pediatrics. 2015.