Place a baby boy and girl side by side and most likely the girl would seek a face to look at, while the boy's eye would be caught by something in motion, like a mobile. This is just one difference between the sexes, many of which begin even before birth.

Beginning at birth, girls are slightly shorter and weigh slightly less than boys. But the differences are much more than that.

"Girls' skeletal systems are more mature, making them slightly more resistant to skeletal injuries," says Kimberly Parker, a registered nurse and early childhood expert at Children's Healthcare of Atlanta. "Boys are not only more physically vulnerable during the first year of life, they are more physically active with behaviors like squirming, kicking and wiggling, which may lead to more accidents."

As the mom of two boys and one girl, Lori Harasem of Lethbridge, Alberta, Canada, noticed considerable differences. "The boys walked sooner and were climbing on things, but I had both assessed at speech therapy because their speech seemed so delayed," says Harasem. "My girl is 18 months and was assessed yesterday and is way ahead in speech. [But] she walked later than the boys and has never climbed like they do!"

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