What is the name of the soft part on the top of a child's head?

Patient's question:

My youngest son just turned one month old. Recently, while bathing him, I noticed that a part of his scalp (excluding the fontanelle) feels soft. When I gently push it with my hand, it feels like there's water accumulating inside, between the scalp and the skull. The bone of the skull is hard, and the area of the soft part is about the size of a child's palm. I would like to ask the experts if they have encountered this phenomenon before. Is it normal? Should I take him to the doctor? Thank you!
First follow-up question: What could be the cause of this? I would appreciate your advice and insights. After all, such a small thing can't speak and just cries when something's wrong. My wife and I are very worried.

Doctor's answer:

Hello, based on the information, neonatal scalp hematoma is generally referring to bleeding between the cranial bone and the periosteum covering it, resulting in blood accumulation. The boundary is clear, and it does not indent when pressed. It becomes noticeable around two to three days after birth and usually disappears within 1 to 3 months.
For the care of a scalp hematoma:
1. Local cold compresses can be applied in the early stages to prevent the hematoma from enlarging.
2. If the hematoma is large, an X-ray should be taken to check for cranial bone damage. Blood tests should be conducted to assess coagulation function, and appropriate treatment should be provided if abnormalities are found.
3. The cranial hematoma should be allowed to absorb naturally. Avoid rubbing the hematoma or drawing blood from it to prevent infection. If infection occurs and forms an abscess, it should be drained promptly through incision.
4. Infants with hematomas should be moved minimally and positioned on their healthy side. If the hematoma is large, temporary artificial feeding may be necessary.
The description of the scalp hematoma appears to be severe. It is recommended to seek active medical evaluation and treatment from the hospital based on the size of the hematoma and the time it takes to resolve.