Diagnosis starts in the delivery room and unfolds across the first weeks of life. Physical exam, imaging, and nerve conduction studies together build the picture, and the sooner that picture comes into focus, the better the outcomes.
The diagnosis is made by watching, not by testing. The American College of Obstetricians and Gynecologists task force describes the presentation in a single line: a weak or paralyzed upper extremity in which passive range of motion exceeds active range.
That distinction is the whole examination in miniature. Someone else can move the arm through its range, and the baby cannot. It separates a nerve injury from a joint that will not move, and it is the reason a careful pair of hands in the first days finds this before any machine does. Neonatal brachial plexus palsy occurs in about 1.5 per 1,000 total births.
What follows is what a proper workup contains and, just as usefully, what it does not decide. The condition itself is covered in our Erb’s palsy overview.
The first examination happens in the delivery room or the nursery, and what it is looking for is asymmetry.
A newborn with an upper brachial plexus injury holds the affected arm limp against the body, shoulder rotated inward, elbow straight, forearm turned so the palm faces backward. The grip is often preserved, which misleads people, because the nerves supplying the hand may be untouched while those supplying the shoulder and elbow are not. The Moro reflex is asymmetric: one arm flings out and the other does not.
Two findings on that first examination change the picture and should be looked for specifically. A droopy eyelid and a small pupil on the same side as the weak arm suggest the nerve roots were pulled from the spinal cord rather than stretched further along, which is a more severe injury. And a clavicle fracture can produce an arm a baby refuses to move for entirely mechanical reasons, so it has to be excluded before weakness is attributed to nerves.
The mechanism is traction on the brachial plexus during delivery, and the causal picture is less tidy than it sounds. The same ACOG task force records that the peer-reviewed literature describes this injury occurring without any clinically recognizable shoulder dystocia, at both vaginal and cesarean delivery. That cuts in two directions at once, and both are worth knowing before anyone treats the mechanism as settled. Our page on Erb’s palsy risk factors goes through what raises the odds and what does not predict it.
One examination establishes that something is wrong. A series of them, scored the same way each time, establishes what happens next.
Recovery is the variable everything turns on, and it can only be seen over months. The instrument used for that in the research literature is the Active Movement Scale, which grades movement at each joint from 0 to 7 in an infant who cannot follow instructions, with scores above 5 counting as functional recovery in the surgical studies. The Narakas classification describes how much of the plexus was involved, and the Mallet score takes over for shoulder function once a child is old enough to copy a posture.
In the order it usually happens:
The specialist question is not usually whether the plexus is injured, which the examination answers. It is where along the nerve, and how much. An injury at the root, pulled away from the spinal cord, behaves differently from a stretch further down, and it is the difference between a lesion that can recover on its own and one that cannot. Horner syndrome, an elevated diaphragm on chest imaging, and the pattern of which muscles are spared all point toward that answer.
The evaluation exists to answer one question with a deadline attached: is elbow flexion coming back, and if not, when should someone operate.
Failure of elbow flexion to recover is the trigger most surgeons use, and the age at which they act on it is genuinely disputed. Across the studies pooled in a meta-analysis of elbow flexion reconstruction, mean age at surgery ranged from 5.7 months to 11.9 months between centers publishing on the same operation. A four-month spread in the mean is the honest answer to how settled the timing question is, and it means asking a surgeon what threshold they use, and why, is a reasonable thing to do.
The second thing a follow-up schedule is for gets less attention. Incomplete recovery leaves the muscles rotating the arm inward stronger than those rotating it out, the arm sits turned in, the humeral head drifts backward out of the socket and the glenoid remodels. That sequence is glenohumeral dysplasia, and it can be measured: researchers reviewing MRI scans of 32 children with this injury quantified humeral head deformity as a ratio of anterior to posterior area, averaging 0.76 on the affected side, and found it tracked with both glenoid version and posterior subluxation.
The same work found the humeral head remodels after joint rebalancing surgery, so a flattened head does not by itself rule out an operation it was once thought to rule out. Range of motion at the shoulder is therefore worth recording at every visit, since losing external rotation is what starts the chain. Physical therapy for Erb’s palsy covers how that range is protected.
Testing supports the picture. It rarely changes the decision, and understanding why saves families a lot of anxiety about numbers.
Nerve conduction studies and electromyography measure how signals travel and how muscles respond, and they are used mainly where the level of the lesion is unclear or where surgery is being planned. MRI shows the plexus and can reveal a pseudomeningocele, the pocket of fluid that marks a root torn from the cord. Ultrasound and plain films handle the clavicle and the diaphragm.
What none of them replaces is the serial examination. The decision to reconstruct rests on whether a specific movement has returned by a specific age, and that is a bedside observation scored consistently over months, which is why the appointment schedule matters more than any single scan.
The diagnosis is usually made in the first days. The prognosis takes until somewhere between six months and a year, because that is how long it takes to know whether the nerve is recovering. Families are often told to wait and see, which is accurate and badly phrased: what is happening in that window is measurement, and it should come with appointments, a scale, and a date by which a decision gets made.
If excessive force or improper delivery technique contributed to your child’s Erb’s palsy, a legal claim can recover the cost of therapy, surgery, and lifetime care. Specialized birth injury lawyers can review your case at no cost.
Weakness or paralysis of an arm caused by injury to the brachial plexus, the nerve bundle running from the neck into the shoulder, usually sustained during delivery. It affects about 1.5 per 1,000 total births, and the defining sign is an arm whose passive range of motion exceeds what the baby moves on their own.
Clinically, by comparing the two arms. Someone else can move the affected arm through its range and the baby cannot, the Moro reflex is asymmetric, and the arm sits limp against the body with the forearm turned in. Testing supports that picture rather than replacing it, and a clavicle fracture has to be excluded because it produces an arm a baby will not move for mechanical reasons.
Traction on the brachial plexus during delivery. The causal picture is less tidy than it sounds: the ACOG task force records that the injury is described in the literature occurring without any clinically recognizable shoulder dystocia, at both vaginal and cesarean delivery. See Erb’s palsy risk factors.
Immediately. The examination that identifies it belongs in the first days, and what takes longer is the prognosis, since that depends on whether the nerve recovers. Ask for follow-up appointments with a scoring scale and a date by which a decision about surgery gets made, rather than an open-ended instruction to wait and see.
The diagnosis itself costs an examination. What follows is where cost accumulates: serial specialist visits through the first year, imaging where reconstruction is considered, and then either therapy alone or nerve surgery followed in many cases by a second shoulder operation.
It sets the clock running on a decision with a deadline. Failure of elbow flexion to recover is the usual trigger for nerve reconstruction, and across studies of that operation the mean age at surgery ranged from 5.7 to 11.9 months, so a child who is not being measured can pass the window while everyone waits.
Yes, in both directions. A clavicle fracture can imitate it, and a preserved grip misleads people into thinking the arm is fine because the nerves to the hand may be untouched while those to the shoulder and elbow are not. In the other direction, one weak arm in a newborn is sometimes attributed to cerebral palsy when the injury is peripheral, which is a different condition with different imaging and different treatment.
Kelsey is an experienced surgical nurse who works extensively with pediatric patients and families navigating birth injuries and cerebral palsy diagnoses. She is licensed by the Missouri Board of Nursing and operates under the Nurse Licensure Compact, allowing her to practice across compact-member states. As both a mentor and patient advocate, Kelsey focuses on safety, communication, and compassionate care: helping families understand medical procedures, treatment options, and the realities of pediatric conditions.
View Kelsey Pabst, RN's full bio →
Our nurses, patient advocates and legal experts are solely focused on bringing you the latest cerebral palsy information, options for financial assistance and access to community support.
Editorial process →The information on this page is provided for general educational purposes and is not a substitute for professional medical or legal advice, diagnosis, or treatment. Always seek the guidance of a qualified physician or attorney with any questions about a medical condition or legal matter. Read our full disclaimer.
