Most common cause of seizure in newborn is –
Hypoxia induced ischemic encephalopathy
Hypocalcemia
Metabolic abnormality
Sepsis
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