When to Lower the Crib Mattress: A Stage-by-Stage Guide

Lower the crib mattress before your baby can sit up unassisted, and lower it to its lowest setting before they can pull to stand. Waiting until you see them do it is waiting too long.

The crib mattress height is one of those safety details that is easy to forget precisely because it worked fine for months. Then one morning you walk in and your baby is sitting up, gripping the rail, looking extremely pleased with themselves, and suddenly the arrangement you set up before they were born is no longer safe.

The principle behind every recommendation here is simple: the mattress should always be low enough that your baby cannot topple over the rail from whatever position they have most recently learned. Because new motor skills tend to appear without warning, the safest approach is to lower it slightly before you think you need to.

The Three Stages of Crib Mattress Height

Most cribs have two to four mattress positions. The exact number varies, but the progression is the same regardless of model.

The highest setting exists for a practical reason: lifting a newborn in and out of a deep crib many times a night is hard on your back, and a newborn who cannot roll or push up is in no danger of going over the rail. It is a convenience setting with a short lifespan.

Stage One: Lower It Before Your Baby Can Sit

The American Academy of Pediatrics advises lowering the mattress before your baby can sit, to a level where they cannot fall out by leaning against the side or pulling themselves over it.

The key word is before. Sitting does not announce itself in advance, and the gap between a baby who cannot sit and a baby who is sitting confidently can be a matter of days. If you wait for proof, you have already spent time in an unsafe configuration.

• Rolling in both directions, or rolling reliably in one.
• Pushing chest off the floor onto straight arms during tummy time.
• Sitting propped up with support, or in a tripod position on hands.
• Rocking on hands and knees.
• Scooting, shuffling, or any deliberate movement across the crib.

Any one of these is enough. They all indicate a baby developing the trunk strength that will shortly let them get upright.

Stage Two: Lowest Setting Before Pulling to Stand

This is the most important adjustment, and the one most often left too late. The AAP guidance is to set the mattress at its lowest position before your child learns to stand.

Pulling to stand changes the geometry completely. A standing baby’s center of gravity is high above the rail, and babies at this stage are enthusiastic, unsteady, and entirely unaware of edges. A fall from standing in a crib onto a hard floor is a serious injury mechanism, and it is the most common way crib falls happen.

• Pulling up on furniture, sofa cushions, or your legs.
• Getting into a kneeling position in the crib.
• Gripping the crib rail and attempting to haul themselves up.
• Cruising sideways along furniture while holding on.
• Standing while holding your hands.

For most babies this window is roughly 7 to 9 months, though plenty of babies pull up earlier. If your baby is crawling with purpose, assume standing is imminent and lower the mattress now.

When the mattress is at its lowest setting, the distance from the mattress surface to the top of the rail should be at least 26 inches on a crib meeting current US safety standards. That depth is what keeps a standing baby contained.

Stage Three: When to Move Out of the Crib Entirely

Eventually the crib itself is outgrown, and there are two clear triggers.

The height rule: move your child to another bed when they reach 35 inches tall, which is about 89 centimetres. This is the standard the AAP cites, and it applies regardless of age.

The proportion rule: the crib rail should be at least three quarters of your child’s standing height, roughly nipple level when they stand. Once the rail sits lower than that, they have enough leverage to tip over it.

And the behavioral trigger, which overrides both: if your child is actively trying to climb out, the crib is no longer safe, whatever their height. A fall from the top of a crib rail is far more dangerous than a fall from a low toddler bed.

There is a middle option worth knowing about. If your child is climbing but is not developmentally ready for the freedom of an open bed, a crib tent is not recommended, but a sleep sack that limits leg movement can buy time by making climbing harder. Many families also move the mattress to the floor as an interim step.

Crib Safety Beyond Mattress Height

While you have the tools out, a few other checks are worth doing at the same time.

• The mattress must fit snugly. No more than two fingers should fit between the mattress and the crib side.
• Crib slats no more than 2 and 3/8 inches apart, about the width of a soda can, so a head cannot pass through.
• Nothing in the crib but a fitted sheet. No bumpers, pillows, blankets, or stuffed toys for babies under 12 months.
• No drop-side cribs. These have been banned from sale in the US since 2011 and should not be used.
• Check hardware regularly. Screws and bolts loosen over time, especially once a baby is pulling and shaking the rail.
• Move the crib away from windows, blind cords, curtains, wall hangings, and anything a child could reach and pull.
• Do not use an aftermarket mattress topper or add extra padding under or over the fitted sheet.

That last point matters more than people expect. Adding a topper raises the sleep surface, which quietly undoes the mattress height you carefully set, and soft padding is itself a suffocation risk in the first year.

If you are using a secondhand or hand-me-down crib, verify it meets the current federal safety standards that took effect in 2011. Cribs manufactured before then may not comply, and older cribs are a common source of recalled hardware.

Practical Tips for Making the Change

Lowering a mattress is usually a fifteen-minute job, but a few things make it smoother.

• Do it during the day, not at bedtime when everyone is tired and the crib is needed imminently.
• Two people is easier than one, particularly for the mattress support on heavier cribs.
• Keep the manual, or find the model number and download it. Bracket configurations differ between cribs.
• Tighten everything afterward, and check again a week later once it has been used.
• Do both sides evenly. An unlevel mattress support creates a gap, which is a genuine entrapment hazard.
• Recheck the sheet fit. A snug fit at one height may be looser at another.

Some babies notice the change and take a night or two to resettle, mostly because the view from the crib is different. This usually passes quickly and is not a reason to delay a safety adjustment.

When to Talk to Your Pediatrician

Crib setup is usually a safety question rather than a medical one, but a few situations warrant a call.

• Your baby has fallen from the crib, even if they seem fine afterward. Head injuries in infants need assessment.
• Your baby is significantly ahead of or behind typical motor milestones and you are unsure how to time changes.
• Your child was born prematurely and you are unsure whether to use corrected age for motor expectations.
• Your child has low muscle tone, a motor delay, or a condition affecting movement, where standard timing may not apply.
• Your child is repeatedly climbing out and you are concerned about injury.

If your baby falls and then vomits, loses consciousness even briefly, is unusually drowsy or difficult to wake, has unequal pupils, or is inconsolable, seek emergency care immediately.

Frequently Asked Questions