It's a new parent's worst fear: Putting a healthy baby down to sleep, and coming back to find him not breathing.

Over the years, researchers have identified some factors that contribute to deaths of infants in their sleep, such as co-sleeping and putting a baby down on his or her stomach to sleep. When a baby's death remains unexplained, it is said to be due sudden infant death syndrome (SIDS).

Now, a new study, published July 14 in the journal Pediatrics, shows that the risk factors that contribute to SIDS vary with a baby's age.

The majority of infants in the study younger than 4 months old who died while sleeping were bed sharing, whereas older infants who died were likelier to be found on their stomachs, or with blankets, stuffed animals or pillows around them, said study co-author Dr. Jeffrey Colvin, a pediatrician at Children's Mercy Hospital in Missouri.

Risk factors

SIDS and unintentional injuries such as suffocation occur in 81 out of every 100,000 live births, according to the National Center for Health Statistics. Doctors now know many risk factors that contribute to what used to be entirely unexplained infant deaths.

Most cases seem to occur when a sleeping baby stops breathing for some reason, but doesn't awaken to change position. Sleep-related deaths are also more common in very young infants, with the risk falling off sharply by the time babies are 6 months old, Colvin said. [7 Baby Myths Debunked]

Stomach sleeping is tied to sleep-related deaths, perhaps because prone infants tend to sleep more deeply, which can make them less likely to wake up if their airway gets blocked or if they stop breathing, Colvin said. And the risk of SIDS is up to 19 times higher for babies when they are used to sleeping on their backs, but wind up on their stomachs for some reason, the researchers wrote in the paper.

Co-sleeping is another common risk factor. A large proportion of SIDS infants are found sleeping next to an adult, whether on a bed, sofa or other surface. Many babies are also found with pillows, stuffed animals, blankets and bumpers in their sleeping area.

Babies who die also tend to have a preexisting vulnerability, such as having trouble rousing quickly, perhaps because there are fewer receptors in their brains to react to normal arousal chemicals, said James McKenna, an anthropologist at the Mother-Baby Behavioral Sleep Laboratory at the University of Notre Dame in Indiana, who was not involved in the new study.

Developmental differences

But less was known about how risk factors change with age. So Colvin and his colleagues studied 8,207 unexplained infant deaths in 24 states between 2004 and 2012.

About 70 percent of the babies who died were three months old or younger, the researchers found.

About 75 percent of the babies who were younger than 3 months old when they died were sharing a bed with another person at the time, compared with about 60 percent of infants 4 months old and older.

Older infants who died were more likely to be found on their stomachs with a blanket, pillow or stuffed animal nearby and about 18 percent of older infants had turned from their backs or sides to their stomachs.

The findings show that the risk factors vary with developmental stage, the researchers said.

The results underscore the importance of putting babies on their backs to sleep, in a sleeping space separate from other people, with no objects in the crib with them, Colvin said. Once babies can roll from back to front typically around 4 months of age they run the risk of ending up on their tummies.

"That's why it's all the more imperative to keep your sleep environment pristine," Colvin said.

In addition, parents should give their babies ample tummy time when awake, to allow them to develop their muscles, and once a baby can roll from back to front, parents who swaddle their babies should leave their arms free, Colvin said.

That way, "when they turn over, they can get those arms to push up a little bit, so they can turn their head to the side," Colvin said.

Bed-sharing and breastfeeding

Having a new baby sleep in bed with another person is dangerous, Colvin said. But the number of families that bed share has stayed stable over the last decade, he added.

That's not surprising, given that babies and caretakers are biologically wired to want to sleep next to each other, McKenna said.

"Human infants are by nature contact-seekers, and particularly within that first few months, when babies are so vulnerable and fragile," McKenna told Live Science.

But because breastfeeding moms often have to nurse their babies several times a night, the recommendations can seem especially unrealistic, McKenna said. Many women who breastfeed can do so without fully waking up, but not if the baby is in a separate sleeping space, he noted.

Given that more than a million babies born each year are already bed-sharing, and that number shows no signs of declining, health professionals should give guidelines to families to help minimize risks associated with bed-sharing, for instance by removing blankets, pillows and soft-sleeping surfaces, McKenna said.

But even among moms who are breastfeeding and do not drink or smoke, co-sleeping quintuples the risk of SIDS, a 2013 study found.

Furthermore, breastfeeding and safe sleeping recommendations don't have to be at cross-purposes. For instance, parents can keep children in a crib right by the bed, or in a co-sleeping bassinet that attaches to the bed, Colvin said.

"It creates a safe space for the baby," Colvin said.

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