The midline
If you hear people talking about ‘crossing the midline’, this is something we do as adults without thinking, but babies need to master this in order to use both sides of their body together.
The ‘midline’ is the centre of the body, so if you imagine a line drawn down the centre of your body from your head to your toes – that's it. We cross the midline when we do things like turning the page of a book or fastening our seatbelt.
Steps towards crossing the midline
The first step towards crossing the midline is when your baby brings their hands together in the centre of their body; they might do this and hold their hands together. They might move their hands and bring them together when moving towards an object held close to them.
The ability to cross the midline gives your baby more places that they can explore. Rather than just looking at and touching objects in front of them, they can reach across to objects on either side of them.[1]
It will allow them to reach more and eventually maintain posture and balance
It is thought that crossing the midline is linked to physical development and to brain development,[1] and as your baby develops their physical skills they will be able to maintain their posture and balance as they reach across their body.
It is likely that when your baby starts to reach they will use both hands together at first. Bringing their hands together in the centre of their body is a step towards using their hands together to reach and explore objects.
Links with other streams
Touching one hand with the other will contribute to your baby's understanding of their body.
Bringing their hands together also links to the development of fine motor skills. Many of the skills that your child will develop in the future will depend on them being able to cross the midline and use their hands together and separately.
When you think about your child writing (one day!), they will hold the paper steady with one hand and move their pen or pencil across the paper from one side to the other, crossing the midline as they write.
Gaining control of their movements and being able to bring their hands together will lead to being able to explore objects with their hands and mouth. This exploration will support your baby’s cognitive development as they think about and plan their movements, and also as they develop their knowledge and understanding of toys and objects.
Your baby's continuing development of movement control
They will reach for objects using both hands at first, then using one arm; they will also develop their posture and be able to reach when sitting. When something captures their interest, you'll see your baby move their arms and cross the midline of their body to reach for them.
References:
[1] Van Hof P., Van Der Kamp J. & Savelsbergh, G.J.P (2003) The relation of unimanual and bimanual reaching to crossing the midline. Child Development. 73 (5) pp 1353-1362.