Boys Just Start Out Bigger

Everyone knows why girls generally don't play on boys' football or basketball teams—they're too small to match the competition. But I always thought that any size advantage in males was due to genetic programming that caused them to grow faster and bigger, starting in toddlerhood.

Wrong. Boys start out bigger in the womb and just keep building on that size advantage after birth.

The revelation comes from a paper by Radek Bukowski of the University of Texas Medical Branch and his colleagues at 10 other U.S. medical institutions, not to mention the University of Cambridge, England and Royal College of Surgeons in Dublin, Ireland.

This august group studied fetal growth in nearly 29,000 babies. Even at just 8 to 12 weeks following conception, boys were bigger than girls. The difference was small, which was why it took so many babies to statistically confirm it was something other than a fluke finding.

Boys retained their subtle size advantage through birth, when they weighed in at some 120 grams more, on average, than girls. That's among boys that were conceived the old fashioned way. However, even among those conceived in a test-tube—and these accounted for 3.5 percent of the babies—boys weighed an average of 90 grams more at birth than girls.

Accounting for mom's height, weight, smoking status, or race didn't alter the trend, the researchers report in the May 15 American Journal of Epidemiology.