A few weeks after the birth of your baby, you notice that one of his arms seems less vigorous, more amorphous. Is this a cause for concern?

In principle, your baby will need to be examined to better understand what’s going on. During this meeting, you’ll be asked about the details of your delivery, and sometimes you’ll be able to make logical, explanatory and revealing connections.

For example, if :

• Difficulty in releasing a shoulder during delivery (shoulder dystocia),
• if there were long pushes for expulsion,
• the use of forceps,
• the baby’s weight at birth (heavier-than-average baby).

What sometimes happens in newborns with one arm more flaccid than the other, with less mobility, is a phenomenon known as perinatal brachial plexus paralysis.

Definition of brachial plexus paralysis

Brachial plexus paralysis is most often defined as an upper limb weakness that is noticeable after birth, with one arm more flaccid and less responsive than the other. The arm remains more hypotonic (less tonus), with less strength, reflexes and movement than the other.

The cause of brachial plexus paralysis

The cause of this paralysis is a lesion of at least one cervical or thoracic nerve root, which impairs the proper functioning of a limb that depends on it.

The brachial plexus involves 5 nerve roots running from the spinal cord to the neck, and then to the extremities: arm, forearm and hand.

All movement and sensory capacities of the upper limbs depend on the good condition of the brachial plexus. If the plexus is impaired, the consequences for certain upper limbs are predictable and vary according to the degree of impairment.

Diagnosing brachial plexus paralysis

The physician will perform a complete physical examination. An X-ray of the shoulder and arm may also be requested to corroborate the observations made during the clinical examination. Subsequently, referral(s) may be made to neurology (to validate the nerve status of the arm concerned), pediatric physiotherapy or occupational therapy (to help regain physical strength and proper functioning of the limb) and/or pediatric osteopathy (to help reposition structures to best support the arm’s mechanics).

Paralysis can occur in the lower part of the arm only, including the elbow, arm and shoulder. But paralysis can also be total (in a lesser proportion – 20 to 30%), meaning that the hand and fingers move very little or not at all.

Why is only one arm affected?

This type of paralysis is most often unilateral. The diagnosis is often obvious at birth, as the newborn is left with one arm unmoved and the other moving without problem. In only around 1% of cases, paralysis occurs in both arms.

Following the diagnosis

Spontaneous recovery is possible and depends on the nature of the lesions, but it is usually impossible to predict how long it will take. Some literature suggests that 75-90% of children will recover on their own within the first month or two of life. A further 20-30% of children may suffer sequelae, depending on the severity of the initial injury. Physical or functional limitations may therefore persist over time.

If after one month the paralysis still seems to be present, a referral to a multidisciplinary team is indicated to set up a suitable treatment plan.

Early and complementary action is desirable to encourage the baby’s maximum physical recovery, for example by means of exercises and specific positioning suggested by the osteopath, occupational therapist and/or physiotherapist.

Marie Fortier
The baby expert

Article updated: March 2025.

References :

• Société canadienne de pédiatrie. (2021). L’évaluation et la prise en charge de la paralysie néonatale du plexus brachial. Repéré le 30 mars 2025 à https://cps.ca/fr/documents/position/paralysie-neonatale-du-plexus-brachial 

• CHU Sainte-Justine. (2011).  Lésion du plexus brachial. Repéré le 30 mars 2025 à https://readaptation.chusj.org/fr/Familles/Conseils-de-nos-professionnels/Sante-et-developpement/Lesion-du-plexus-brachial

• Centre intégré de santé et de services sociaux de la Montérégie-Ouest. (2024). Aide-mémoire guichet d’accès DI-TSA-DP services spécialisés externes en déficience physique, Atteinte périnatale du plexus brachial. Repéré le 30 mars 2025 à https://www.santemonteregie.qc.ca/sites/default/files/2024/03/atteinte_perinatale_plexus_brachial_aide-memoire_referent.pdf