Physiologic Anemia of Infancy: This condition occurs due to the natural decrease in red blood cell mass after birth, as the infant's body adapts to extrauterine life. It typically presents around 2-3 months of age but can start as early as 6 weeks.

Iron Deficiency Anemia: Although less common at 6 weeks, infants who were premature, had low birth weight, or are fed with cow's milk early on might start to show signs of iron deficiency anemia earlier than term infants.
Vitamin Deficiency (e.g., Vitamin B12 or Folate Deficiency): Though rare at this age, maternal dietary deficiencies could potentially lead to deficiencies in the infant, especially if breastfeeding.

Hemolytic Disease of the Newborn (HDN): Caused by maternal antibodies against the infant's red blood cells, leading to their destruction. It's crucial to identify and treat promptly to prevent severe anemia and kernicterus.
Infection (e.g., Congenital Infections): Certain infections, such as malaria (in endemic areas), toxoplasmosis, or other congenital infections, can cause anemia. Early detection is vital for appropriate management.
Blood Loss: Internal or external bleeding, which could be due to various reasons including trauma, vascular malformations, or iatrogenic causes, needs to be identified and managed immediately.

Congenital Red Blood Cell Disorders (e.g., Sickle Cell Disease, Thalassemia): These genetic disorders can cause anemia and are crucial to diagnose early for proper management and genetic counseling.
Congenital Bone Marrow Failure Syndromes (e.g., Diamond-Blackfan Anemia): Rare conditions characterized by a failure of the bone marrow to produce sufficient red blood cells, which can present in early infancy.
Gastrointestinal Anomalies Leading to Blood Loss: Conditions like intestinal duplication or Meckel's diverticulum can cause occult blood loss leading to anemia.