Diaper Dermatitis (Diaper Rash): This is the most common cause of a rash in the diaper area, extending to the belly. It's often due to irritation from stool and urine, friction from the diaper, and sometimes a fungal or bacterial infection. The rash is typically red, painful, and may have small bumps or pimples.

Atopic Dermatitis (Eczema): Infants with atopic dermatitis may develop a rash that starts in the diaper area and spreads to the belly. It's characterized by dry, itchy, and scaly skin.
Contact Dermatitis: This can occur due to an allergic reaction to diaper materials, soaps, or lotions, causing a rash that may spread from the diaper area to the belly.
Impetigo: A contagious bacterial infection that can cause a rash, often starting in the diaper area and potentially spreading to other parts of the body, including the belly.

Staphylococcal Scalded Skin Syndrome (SSSS): Although rare, SSSS is a serious condition caused by staphylococcal toxins that can lead to widespread skin blistering and peeling, starting in the diaper area and potentially spreading to the belly. It requires immediate medical attention.
Herpes Simplex Virus (HSV) Infection: HSV can cause a rash in the diaper area that may spread. It's crucial to identify and treat HSV infections promptly to prevent serious complications.

Zinc Deficiency (Acrodermatitis Enteropathica): A rare genetic disorder that leads to zinc deficiency, causing skin lesions, including a rash that might start in the diaper area and spread to the belly.
Langerhans Cell Histiocytosis: A rare disorder in which the body accumulates too many immature Langerhans cells, leading to various symptoms, including skin rashes that might appear in the diaper area and spread.
Congenital Syphilis: A condition that occurs when a baby is born with syphilis, a bacterial infection usually acquired from the mother during pregnancy. It can cause a variety of symptoms, including rashes.