Hemolytic Disease of the Newborn (HDN) due to ABO Incompatibility: This condition occurs when there is an incompatibility between the blood types of the mother and the baby, leading to the breakdown of the baby's red blood cells. Given the baby's A+ blood type and the mother's O+ blood type, ABO incompatibility is a likely cause of the jaundice, as the mother's immune system may produce antibodies against the A antigen on the baby's red blood cells.

Physiological Jaundice: This is a common condition in newborns, peaking around the 3rd to 5th day of life, but it can start as early as 24 hours. It's due to the breakdown of red blood cells and the liver's initial inability to process all the bilirubin being produced.
Breastfeeding-Associated Jaundice: Although less common, breastfeeding can sometimes contribute to jaundice, either through inadequate milk intake leading to dehydration and concentrated bilirubin or through substances in the breast milk that can increase bilirubin levels.

Biliary Atresia: A serious condition where the bile ducts are blocked, leading to a build-up of bilirubin. Early diagnosis is crucial for effective treatment, which often involves surgical intervention.
Infections (e.g., Sepsis, Urinary Tract Infections): Infections can cause jaundice in newborns and are critical to identify and treat promptly to prevent serious complications.
Congenital Thyroid Disorders: Hypothyroidism can cause jaundice, among other symptoms, and is essential to diagnose early to initiate appropriate treatment and prevent long-term developmental issues.
Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency: This genetic disorder can lead to hemolytic anemia and jaundice, especially in response to certain medications or infections.

Congenital Spherocytosis: A rare genetic disorder affecting the red blood cell membrane, leading to hemolytic anemia and jaundice.
Other Red Blood Cell Membrane Disorders: Similar to congenital spherocytosis, these are rare genetic conditions that affect the structure of red blood cells, leading to their premature breakdown.
Galactosemia: A rare genetic disorder that affects the body's ability to metabolize the sugar galactose, which can lead to jaundice among other severe symptoms.
Pyrrole Disorders (e.g., Acute Intermittent Porphyria): Extremely rare genetic disorders affecting the production of heme, a component of hemoglobin, which can lead to neurological symptoms and jaundice, though these are more commonly seen in older children and adults.