Two weeks ago, I noticed that my son's umbilical cord looked like an inflated ball, and it was outside his tummy/ and it was sticking out of his belly button. I guessed that it was an umbilical hernia. When a health visitor saw him last week, she confirmed my suspiciom that my son has an umbilical hernia. She reassured me that umbilical hernias are very common among newborns and will go away by themselves when they are about five years old. Note > Commas are used / among newborns > so use plural "are"/ used the "past tense"because you talking about something from the past , except for what the nurse told you : she is talking about a fact that is still true in the present / capitalisation not required/ your son "has" is correct since the problem still persist.

Two weeks ago I noticed that my son's umbilical cord had the appearance of an inflated ball and was outside his tummy. I guessed that it was an umbilical hernia. The rest of your sentences read naturally to an English speaker. As you can see, I have just made minor changes. Be careful with where you use capitalization - for example - cord and not Cord.