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As adults, we cross the midline all the time without thinking about it — when we flip through the pages of our favorite parenting magazine, tie one of our shoes, or reach across our body to pull down or fasten our seatbelt.

For babies, the motion isn’t quite as second nature. Here’s what to expect in terms of when your baby will cross the midline and how you can encourage this milestone.

Visualize a paper doll cutout. Fold it in half so that the left half is lying on top of the right half. Now open it up. The fold you made going down the center of the body is the midline.

Crossing the midline happens when your child moves their hand or foot across this line to work on the opposite side of their body.

Before crossing the midline happens, a child will typically use only one side of their body at a time. For example, they’ll use their left hand only to play with a block on their left side.

So, when do babies start to master this motion? Pretty early.

Crossing the midline starts as soon as your baby begins reaching for objects with two hands, at around 4 months of age. But if your little one isn’t there yet, don’t worry — there’s a range of normal.

Some infants may start reaching at about 2 months, while others may mosey their way to the milestone at around 6 months.

This ability to cross the midline continues to develop until your child reaches 8 or 9 years old. At this age, they usually have sufficient core stability and also use both sides of their body.

Crossing the midline should now be a fully integrated skill.

As your baby develops, they’ll most likely begin to cross the midline spontaneously. And unless you’re watching out for it, you may miss this milestone.

Here’s the heads-up on what to watch for.

Around 4 months

Hold a rattle or a brightly colored toy slightly off-center, and your baby may cross the midline to reach it.

Around 9 months

Crawling, rolling, or scooting around helps your baby discover the big, wide world.

Toys (as well as bottle caps, hair pins, and the potato peelings that missed the counter) will attract their attention. Your baby may stretch across the midline to reach them.

Around 10–12 months

Self-feeding encourages your child to cross the midline. This happens spontaneously, when one hand is already holding food and they have to reach across the midline with their free hand.

As your child develops and begins to interact with their environment, they’ll naturally cross the midline.

Here are some fun ways to encourage them.

Activities for babies

  • Start by encouraging your baby to bang at something held at the midline, like a tambourine.
  • Give your baby plenty of tummy time, with toys scattered in an arc in front of them to encourage them to reach out.
  • Lay baby on their back, then help them use their hands to reach for the opposite foot.
  • Put stickers on the back of one hand and have them peel them off with the other hand.
  • Play simple games like patty cake.
  • Encourage your child to use both hands to reach for a large ball that you hold off-center. Research shows that using both hands to reach for an object helps a child learn how to cross the midline.

Activities for toddlers

  • Self-care like scrubbing down in the bath, brushing teeth, and combing hair all involve crossing the midline. And you’ll be working on those essential life skills to boot!
  • Play Simon Says and make sure the movements cross the midline.
  • Drawing large figure eights is a favorite with therapists. Use large sheets of paper to draw the number, then have fun racing cars on your track. At the beach, you can do the same activity in the sand.
  • Have your child toss bean bags or balls towards targets that you place in front of them but off-center.
  • Ditto with kicking balls. You may discover that you have a soccer star in training!

Because children find ways to compensate for what they can’t easily do, it isn’t always easy to immediately notice when a child has trouble crossing the midline.

Here are some things could indicate a delay.

Younger children

  • Some babies can’t quite put together the coordination skills necessary to crawl. Later on, they may have trouble with other gross motor (physical) skills like skipping or doing jumping jacks. Note, though, that some babies just crawl late — or skip crawling altogether — and this doesn’t necessarily mean there’s a problem.
  • Does your child consistently use their right hand to reach for things on their right side and their left hand to reach for things on their left side?
  • Some children avoid crossing the midline to reach for objects on the opposite side of their body. Instead, they rotate their entire trunk to reach the objects without crossing the midline.

Older children

  • Age-appropriate self-care tasks can become tricky when you can’t cross the midline. Children who can’t cross the midline may show reduced independence when faced with tasks like brushing their hair.
  • Some children will lean way over to one side when drawing or even writing to avoid crossing over the midline. Alternatively, they may shift their project over to the side to reach it more easily.
  • Children who consistently switch hands during an activity like cutting have found a way to avoid crossing the midline.
  • Most children will discover that they prefer kicking a ball with their dominant foot, but children who can’t cross the midline may switch feet easily.

Difficulty with crossing the midline doesn’t, in and of itself, indicate a specific medical problem. But as a neurodevelopmental soft sign, it may be one of several developmental abnormalities seen in children with certain conditions.

And crossing the midline is important. Without this nifty ability, both your child’s hands get equal practice.

Handedness itself — that is, whether your child is right-handed or left-handed — is established early, but the ability to best use the dominant hand might be affected if your baby has problems crossing the midline.

Without a strong preferred hand, your child’s fine motor skills may be delayed. That means, among other things, poor handwriting skills and poor performance in sports. These challenges may affect your child’s self-image.

Crossing the midline means using both sides of the body, and that takes extra processing time. By engaging your child in activities that encourage crossing the midline, you may also be helping to train their brain.

While it’s hard to believe it when gazing into your child’s cherubic face, crossing the midline is probably only the first of many lines that they’ll cross on their journey to adulthood.

If you suspect that your child has difficulty crossing the midline, speak with your pediatrician to find out if your little one needs help like occupational therapy.