If you’re putting a baby to sleep, always remember your ABCs: Alone, on their Back, in a Crib.

Placing babies on their backs to sleep is one of the best ways to prevent sudden infant death syndrome (SIDS), the leading cause of death among babies under age 1 in the United States. In fact, ever since the American Academy of Pediatrics launched its first “Back to Sleep” campaign in the early 1990s, the rates of SIDS have declined by more than 50 percent.

Experts aren’t entirely sure why babies who sleep on their stomachs are at a higher risk for SIDS, says Rebecca Dudowitz, M.D., an assistant professor in general pediatrics and the vice-chair of the Primary Care College in the David Geffen School of Medicine at UCLA, but there are a few theories.

One hypothesis: Babies who sleep on their stomachs may breathe in less oxygen because they are “re-breathing their own air, which has more carbon dioxide in it,” says Dr. Dudowitz. It’s also suspected that because babies’ nervous systems are still developing, they may be less likely to wake up from a deep sleep if they’re having trouble breathing, she explains. 

“Even if we don’t understand the mechanism for certain, multiple studies have shown that sleeping on the tummy is a very strong risk factor for SIDS and that sleeping on the back is protective [against it],” Dr. Dudowitz says.

In addition to placing your baby on their back to sleep, here are a few other ways to ensure a safe sleeping environment for your little one.

1. Make sure the bed is firm

Only put your baby to sleep in a crib, bassinet, or play yard that meets the Consumer Product Safety Commission safety standards—not a mattress made for adults or a couch cushion. “One reason why we think soft bedding is dangerous is because of the potential for there to be air pockets,” says Dr. Dudowitz. (Air pockets are thought to “trap” a baby’s re-breathed air, causing them to breathe in less oxygen and more carbon dioxide.)

Babies also shouldn’t fall asleep sitting upright, she says, since they might slump into a position that obstructs their airway. “There have been a number of sleeping devices recalled because they placed babies at an incline,” she says.

2. Share a room (not a bed) with baby

Putting your baby’s crib or bassinet in your room (known as “room sharing”) allows you to keep a close eye on your baby. In fact, the AAP says that it can decrease the risk of SIDS by as much as 50 percent. Plus, says Dr. Dudowitz, “babies tend to sleep better that way.” The AAP recommends sharing a room with your baby for at least the first six months, and ideally, for one year.

“Room sharing” isn’t the same thing as “bed sharing,” however. Sharing a bed with your baby isn’t recommended by the AAP because it can put babies at risk for suffocation.

3. Keep the crib clean

Speaking of cribs, ditch the pillows, blankets, and even teddy bears. “Babies wriggle around in their sleep and can become entangled in blankets,” says Dr. Dudowitz. “Or the blanket or crib bumper can simply restrict air flow around the face, again leading to re-breathing.” The solution for cold nights? Dress your baby in sleep clothing such as a one-piece sleeper or a sleepsack.

Here’s another letter to remember if your baby is fussing on their back: “S.” During the first three months of your baby’s life, Dr. Dudowitz recommends using the five “S’s:” swaddling, making the “shhh” sound, gently swishing them back and forth, giving them a pacifier to suck on, and holding them on their side or stomach. (Keep in mind that once a baby starts trying to roll over from back to front, which could happen around 2 months of age, it’s not safe to swaddle them anymore.)

“Then, once they are asleep, you can lay them on their back,” she says. The AAP recommends putting babies on sleep on their backs for the first year.