Dear Doctor K: The other day I swaddled my niece before putting her down for her nap. My sister told me that's dangerous. Really?

Dear Reader: My colleague Dr. Claire McCarthy, assistant professor of pediatrics at Harvard Medical School, confirmed that your sister is right.

To swaddle a baby is to wrap cloth (such as blankets or large towels) tightly around the baby, making it harder for the infant to move his or her arms and legs. My mother said that when I was an infant, I used to bang on the bars of my crib with my hands and feet. I guess, even at 6 months old, I longed for freedom and independence. Or maybe I was just ornery. In any event, when I got that way, she swaddled me.

Perhaps you have swaddled your babies and with no ill effects. In fact, swaddling has been part of caring for babies for centuries — millennia, really. It makes a baby feel like he's back inside the womb, or being snuggled close. It calms many babies and helps them sleep better.

It also can really help some parents get their babies to fall and stay asleep on their backs. And that's what pediatricians recommend to help prevent sudden infant death syndrome, or SIDS, which is more likely when babies sleep on their stomachs. Babies who are swaddled are also less likely to startle themselves awake.

But there are down sides to swaddling. Because swaddling keeps the legs together and straight, it can increase the risk of hip problems. And if the fabric used to swaddle a baby comes loose, it can increase the risk of suffocation.

The most recent warning about swaddling comes from a study published in the journal Pediatrics. Although swaddling can help babies to sleep on their backs, that's not what parents sometimes do. The study found that when swaddled babies were put on their sides or bellies, instead of their backs, their risk of SIDS went up a lot. For those put on their bellies, especially babies more than 6 months old, the risk doubled.

The study can't tell us exactly why the risk doubled. But one can imagine that a tightly swaddled baby might not be able to get her head up if she started having trouble breathing. And if that swaddling blanket came loose and she was face down, it also might make smothering more likely.

Here's what parents should consider when they think about swaddling:

Always put your baby to sleep on his back. This is true no matter what, but is especially true if he is swaddled.

Make sure that whatever you are using to swaddle can't come loose.

Babies' legs need to be able to bend up and out at the hips for healthy development. If your baby is going to spend a significant amount of time swaddled, use a swaddling sleep sack that lets the legs move. It may not be quite as effective in calming the baby, but it is safer for the baby's hips.

Dr. Komaroff is a physician and professor at Harvard Medical School.