You should lower the crib mattress based on your baby’s physical abilities, not their age. The first drop to the middle setting happens around 6 months, when your baby starts sitting up or attempting to crawl. The second drop to the lowest setting should happen by 9 months, or whenever your baby begins pulling up to stand.
Why Age Isn’t the Right Marker
Babies hit motor milestones on wildly different timelines. Some start pulling to stand at 7 months, others not until 11. Pediatrician Laura O’Connor at Cleveland Clinic puts it simply: “The more mobile your baby gets, the lower the crib should go.” That means watching what your baby can do matters far more than counting months on a calendar. If your 5-month-old is already getting up on hands and knees, it’s time to lower the mattress, even if you weren’t planning to do it yet.
Roughly 10,900 emergency department visits per year involve cribs and crib mattresses among children under five, and falls are the leading cause of all nursery product injuries. Most of these are preventable by adjusting the mattress height before a baby gains the ability to lean or climb over the rail.
The Highest Setting: Newborn Only
Most cribs come with three mattress positions. The highest setting is designed for newborns who can’t yet roll, sit, or push up. Federal safety standards require at least 5 inches between the top of the mattress at its highest position and the top of the lowest rail, which provides a minimal barrier. That’s enough for a baby who stays on their back, but it becomes dangerous fast once any movement starts. Many parents keep the mattress here for the first four to five months, though you should lower it sooner if your baby rolls over early or starts pushing up on their arms during tummy time.
The Middle Setting: Sitting and Pre-Crawling
Drop to the middle position before your baby can crawl. The key abilities to watch for are:
- Sitting up unassisted, even briefly
- Commando crawling, pulling forward on their belly using their arms
- Getting onto hands and knees, even if they rock in place without moving
For most babies, at least one of these happens around 6 months. The important thing is to make the change before your baby masters the skill, not after. If you notice your baby rocking on hands and knees in the crib during a nap, lower the mattress that same day. Babies often surprise parents by advancing quickly from rocking to full crawling within a week or two.
The Lowest Setting: Pulling to Stand
Once your baby can pull themselves up to standing, the mattress needs to be at its lowest position. At this level, federal standards require at least 22 inches from the mattress surface to the top of the rail. That distance is what keeps a standing baby safely contained.
Pulling to stand typically happens around 9 months, but some babies do it closer to 7 or 8 months, especially if they practice on furniture during the day. A good rule of thumb: if your baby grabs the crib rail and gets to their knees, assume standing is days away. Go ahead and make the adjustment.
This is the setting your child will stay on for the remainder of their time in the crib, which for most kids lasts until somewhere between age 2½ and 3.
How to Make the Adjustment Safely
Lowering a crib mattress usually involves removing the mattress, unscrewing or unclipping the support brackets from one set of holes, and reattaching them at a lower set. The process takes about 10 to 15 minutes with a screwdriver or Allen wrench (most cribs include one). A few things to check each time you make a change:
- Tighten every bolt and screw. Loose hardware can allow the mattress support to shift or detach, especially when a toddler jumps or bounces.
- Confirm all four corners sit at the same height. An uneven mattress platform can create a gap between the mattress and the crib frame.
- Give the frame a firm shake after reassembly. Nothing should wobble or click.
- Check your owner’s manual. If you’ve lost it, most manufacturers post PDFs on their website. Using the wrong bracket holes or skipping a step can compromise the structure.
Signs It’s Time to Leave the Crib Entirely
Even at the lowest mattress setting, your child will eventually outgrow the crib. Three signals tell you it’s time to transition to a toddler bed or regular bed:
- Your child is 35 inches tall. At this height, most crib rails no longer provide a safe barrier. Many crib manufacturers set 35 inches (and 50 pounds) as the upper limit.
- The rail hits mid-chest. If the top of the rail lines up with the middle of your child’s chest when they stand inside the crib, their center of gravity is high enough to tip over the edge.
- Your child climbs out. Some toddlers attempt this before their first birthday, while others never try. Once a child successfully gets a leg over the rail, the crib is no longer safe regardless of the mattress position.
One thing that can delay this transition: avoid teaching or encouraging your toddler to climb into their crib. Kids who learn to climb in quickly figure out they can climb out, too. Lifting them in and out yourself keeps the crib feeling like a contained sleep space for longer.
If You’re Unsure, Go Lower
When in doubt, always choose the lower setting. There is no downside to having the mattress too low. It just means you bend a little further to lay your baby down. A mattress that’s too high, on the other hand, is a fall waiting to happen. Babies practice new skills in the crib at night and during naps, often before they’ve demonstrated those skills during the day. The safest approach is to stay one step ahead of your baby’s development rather than reacting after they’ve already surprised you.