Vertebrae is the series of small bones that make up the spinal column. Girls are born with weaker vertebrae and boys are born with stronger spines, says a new study.
The researchers found that vertebral cross-sectional dimensions, a key structural determinant of the vertebra's strength, were 10.6 percent smaller on average in newborn females than in males.
"Human beings are the only mammals in which this difference is seen, and it is one of the few key physiological differences between the sexes," said senior study author Vicente Gilsanz from The Saban Research Institute of Children's Hospital Los Angeles, US.
Results of the study published online in the Journal of Pediatrics
suggest that this difference is evolutionary, allowing the female spine to adapt to the fetal load during pregnancy.
But this difference in vertebrae makes women more vulnerable to scoliosis - an abnormal curving of the spine and the bone disease osteoporosis, the study said.
In the study, 70 healthy, full-term newborns (35 boys and 35 girls) were measured. Weight, body length, and head and waist circumference did not significantly differ between sexes.
Compared to newborn boys, girls had significantly smaller vertebral cross-sectional dimensions - a difference independent of gestational age, birth weight and body length.
"Although we have known that girls had smaller vertebrae than boys, we did not know how early this difference first occurred," Gilsanz noted.
The study noted that, over their lifetimes, women also accumulate less bone mass than men, resulting in a two- to four-fold increase in spinal fracture.
"While girls are born with a predisposition to developing osteoporosis as older adults, we know that bone development can be optimized with exercise and nutrition," Gilsanz pointed out.