Neonatal Abstinence Syndrome (NAS): Given the mother's history of taking sertraline, morphine, and codeine, along with smoking, the baby is at high risk for NAS, which can cause tremors among other symptoms like irritability and hyperreflexia.

Hypoglycemia: Newborns, especially those with risk factors like maternal diabetes or those who are premature or small for gestational age, can experience hypoglycemia, leading to tremors or jitteriness.
Hypocalcemia: Low calcium levels can cause neuromuscular irritability, leading to tremors or seizures in newborns.
Opioid Withdrawal: Directly related to the mother's use of opioids (codeine), this condition can cause tremors, along with other withdrawal symptoms.

Intracranial Hemorrhage: Although less likely, given the vaginal delivery at 38+1 weeks of gestation, an intracranial hemorrhage could cause tremors or seizures and is critical to diagnose promptly due to its severe implications.
Infection (e.g., Meningitis or Sepsis): Infections in newborns can present with non-specific symptoms, including tremors or jitteriness, and are medical emergencies.
Congenital Neurological Disorders: Certain congenital conditions affecting the brain or nervous system could present with tremors, though these would be less common and might be associated with other signs or a family history.

Congenital Thyroid Disorders: Hypothyroidism or hyperthyroidism can cause neurological symptoms, including tremors, though these conditions are relatively rare and usually associated with other systemic signs.
Inborn Errors of Metabolism: Certain metabolic disorders can present in the neonatal period with neurological symptoms, including tremors, but these are rare and often come with other systemic signs or specific laboratory findings.
Benign Neonatal Sleep Myoclonus: A rare condition characterized by myoclonic jerks during sleep, which could be mistaken for tremors, though it's benign and resolves spontaneously.