Boys and girls have subtly different spines, and the difference is present at birth, according to a new study out of Children's Hospital Los Angeles. Using magnetic resonance imaging (MRI) on 35 full-term newborn girls and 35 full-term newborn boys, researchers report in the Journal of Pediatrics that vertebral cross-sectional dimensions, which help determine the vertebra's strength, are roughly 11% smaller in newborn girls than in boys—and the difference is independent of gestational birth weight or body length. What's more, they say the difference isn't found in any other mammals, and it's one of the only physiological differences between the sexes in humans at birth.
"Although we've known that girls had smaller vertebrae than boys, we did not know how early this difference first occurred," one researcher says in a press release. "Our study ... provides new evidence that this difference begins during prenatal development." The researchers surmise that the smaller cross-section dimensions allow the female spine to adapt to the fetal load while pregnant. But there's a down side. Women also accumulate less bone mass than men over the course of their lives, leading to more spinal fractures and an increased risk of scoliosis and osteoporosis. Still, "bone development can be optimized with exercise and nutrition," one researcher says. (When it comes to birth order, firstborns have higher IQs ... but there's a catch.)