Most common cause of seizure in newborn is –
DIFFERENTIAL DIAGNOSIS OF NEONATAL SEIZURES 

Hypoxic ischemic encephalopathy → Most common cause (60%).
Intracranial hemorrhage                       
Hypoglycemia                                   
Hyponatremia                                     
Pyridoxine deficiency                         
Drug withdrawal                                 
Benign familial neonatal seizures
Infection
Hypocalcaemia, Hypomagnesaemia
Disorder of amino and organic acid metabolism, hyperammonemia
Developmental defects
No cause found
Hypoxia induced ischemic encephalopathy
Hypocalcemia
Metabolic abnormality
Sepsis