Urogenital trauma: This is often the most common cause of vaginal bleeding in young children, which can result from accidental falls onto objects, sexual abuse, or other forms of trauma. The age and vulnerability of a 1-year-old make this a plausible explanation.

Vaginal foreign body: Young children may insert small objects into their vagina, leading to irritation, infection, and bleeding. This is a common issue in pediatric gynecology.
Infection: Bacterial or fungal infections can cause vaginal discharge and bleeding in toddlers. These infections can be due to poor hygiene or the introduction of bacteria from the gastrointestinal tract.
Urethral prolapse: Although less common, urethral prolapse can cause bleeding and is more frequently seen in young girls, especially those with constipation or who strain during urination.

Child abuse: While not the most common cause, it is crucial to consider and investigate any possibility of sexual abuse in cases of unexplained vaginal bleeding in children. Missing this diagnosis could have severe and long-lasting consequences for the child.
Tumor or malignancy: Although rare, tumors such as sarcomas (e.g., rhabdomyosarcoma) can present with vaginal bleeding in young children. Early detection is critical for effective treatment.

Bleeding disorders: Conditions like von Willebrand disease or other coagulopathies can present with vaginal bleeding, though this would be unusual as a first presentation in a 1-year-old without a known family history or other symptoms.
Hormonal influences: Rarely, hormonal imbalances could lead to vaginal bleeding in young children, but this would typically be associated with other signs of hormonal disturbance.