From day one, your newborn comes wired with all sorts of reflexes — movements that look pretty weird but are actually a sign of how your baby’s brain and nervous system are developing.
Some of these reflexes come as a response to something you do (like put your finger in your baby’s pint-sized palm), while others are spontaneous (like those jerking movements when babies doze). One of the stranger reflexes you’ll see is called the tonic neck reflex.
What is the tonic neck reflex in babies?
You might miss the tonic reflex, more formally known as the asymmetric tonic neck reflex (ATNR), because it can be subtle. When you place your newborn on his back, his head will turn to one side. At the same time, the arm and often the leg on that side will extend, with the hand slightly open, while the opposite arm bends at the elbow, with the hand closed.
That’s why the tonic neck reflex is also known as the fencing reflex because babies look like they’re ready to grasp a sword and say, “En garde!” whenever they do it.
If your baby turns his head to the other side, the same thing will happen in reverse. The arm on that side extends and the opposite arm bends at the elbow.
One caveat: Babies need to be relaxed (say, dozing) in order for this reflex to appear. If your baby is crying or otherwise distressed, he may not assume this fencing position.
When does the tonic neck reflex appear?
The tonic neck reflex will most likely be there from day one, though it can take a month or so for that fencing posture to appear. Interestingly, the asymmetric tonic neck reflex actually develops during the second trimester, by around week 18 of pregnancy.
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When does the tonic neck reflex disappear? Anytime between 4 and 7 months.
You might be wondering if there’s any reason why babies have a tonic neck reflex. Experts think it may prepare babies to develop the muscle tone they need to reach for things when they’re around 4 months old.
How do you test for tonic neck reflex?
Your baby’s doctor will test for the tonic neck reflex while testing for all the other hard-wired newborn responses. This will happen during your newborn’s first checkup at the hospital as well as your baby’s checkups with the pediatrician.
So what’s the pro secret on how to elicit tonic neck reflex? While your baby is lying down, the pediatrician will gently turn your newborn’s head to one side and check if the one arm extends while the opposite arm bends. Then the provider will turn your baby’s head to the other side to see if the same movement occurs.
You can check the tonic neck reflex too. Just make sure that your newborn is lying down and in a relaxed, alert state. Then very gently turn her head to see her strike that fencing pose. Note if anything seems unusual (see below) but remember you may get a different response than the doctor. Try again some other day. And if you’re worried, bring it up at your next well-baby checkup.
When to call the doctor
New-baby reflexes are important because they show doctors that your newborn’s brain and nervous system is working well. That’s why pediatricians check the tonic neck reflex as well as other newborn responses until they’re supposed to go away.
It also makes sense for you to keep an eye on your baby’s reflexes too. Things to watch out for:
The fencing reflex should occur equally — meaning when your baby turns her head to the left, the left arm extends and the right arm bends slightly and when she turns right, her right arm extends and the left arm bends. If one side or the other looks different, tell the doctor.
If your baby seems to move one side of her body better than the other, let the pediatrician know.
If the tonic neck reflex goes away for a while and then returns, tell the doctor. It may signal a problem with your baby’s nervous system.
If the fencing reflex disappears before around 4 to 5 months or lingers well past 7 months and even into toddlerhood, let your doctor know.
While it’s not entirely clear what the original purpose of the tonic neck reflex was, some experts speculate it was Mother Nature’s way of ensuring that babies stayed close to their moms and didn’t roll away when they were put on their backs. That’s something to smile about the next time you catch your little fencer snoozing in her crib.
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