This is the most likely diagnosis given the above physical exam and risk factors, including maternal gestational diabetes and macrosomia.

Erb',s palsy is a C5 and C6 injury that accounts for the majority of obstetrical brachial plexus nerve palsies in newborns.

The etiology of these nerve palsies may be iatrogenic lateral traction on the fetal head, usually during deliveries in which there is shoulder dystocia.

Arm weakness involves the deltoid and infraspinatus muscles (mainly C5) and biceps (mainly C6). On physical exam, the upper arm is adducted and internally rotated with the forearm extended, but the hand and wrist are spared.

Klumpke',s palsy (C8-T1 nerve injury) is not correct since this is seen much less frequently and often presents with hand paralysis and ipsilateral Horner',s syndrome.

Clavicular fractures and humerus fractures are less common fetal complications of shoulder dystocia.

Diaphragmatic paralysis would occur with injury to the phrenic nerve.

However, this would present with signs and symptoms of respiratory distress including tachypnea, mild to severe respiratory distress, cyanosis, apnea, or a weak cry.

Neonates with diaphragmatic paralysis may also have a brachial plexus paralysis, however given that this baby has no respiratory symptoms, this is a less likely diagnosis.