My 4 month old son has a swelling on his head. he was delivered by vaccum. a blood work was done and its all normal. what is it and why did it happen?

Likely OK: Vacuum extraction can occasionally lead to a blood pocket forming under the scalp that will take months to resolve.It may initially be soft & then form a calcium deposit on the skull. Over time the normal growth & remodeling of the skull will let it fade.There is a smaller chance it could be a collection of blood vessels within the scalp, but that should be evident on physical exam.