Expert answer: The BabyCenter Editorial Team
Yes, scary and exasperating though it is, this behavior is common – and perfectly normal – in babies and toddlers.
"Virtually all babies go through this phase," says Bob Sears, a pediatrician in San Clemente, California, and the author of many parenting books, including The Vaccine Book: Making the Right Decision for Your Child. (He's also the son of pediatrician and attachment parenting guru William Sears.)
Sears attributes the classic arch-and-fling to frustration and uncontrollable emotions, which can overwhelm your child's sense of safety and control of his or her body.
"So even though it's not safe for your child to launch herself backward out of your arms, she doesn't have enough control to stop herself," Sears says.
Many parents notice their children doing this somewhere around the 9-month mark. Sears says the trigger is emotional ("probably just the ability to become frustrated"), not physical.
With a growing perceptiveness, a 9-month-old can envision the way he or she wants things to be and feels angry when they don't go that way. You may never figure out what prompted your child to arch. It may be something as minuscule as you handing over the wrong toy or singing the wrong song.
Whatever the cause, you'll want to beware when your child begins hurling himself or herself out of your arms, and take extra care in picking up your child in mid-arch. "Be ready to hold on tight," advises Sears. "You can also quickly set your baby down on a soft surface and allow her to throw herself backward on a soft carpet, pillow, or mattress."
Don't worry – back arching is very unlikely to be a sign of a medical problem. Seizures, a rhythmic shaking of the limbs, look very different from an arch of frustration. There's no confusing the two, says Sears.
However, back arching that occurs exclusively after your baby eats, often accompanied by gas, turning the head to the side, flexing the elbows, and extending the hips, can be a sign of gastroesophageal reflux (GERD). This relatively rare bodily reaction is known as Sandifer's syndrome. Sandifer's is often mistaken for seizures. Talk to your pediatrician if you suspect seizures or reflux.
But if your child is like most, the back-arching sessions signal nothing more than your child's growing independence and signify that his or her emotional development is right on track. So brace yourself and hang on: This won't be the last time you'll have to remain calm as your child rages out of control.