Generally, hematomas such as this will be absorbed back into the body eventually. The only worries would be if infection set in or if it got bigger and put pressure on the brain. I would ask her doctor about the chances of it causing undue pressure, and ask them if and how they plan to monitor it until it goes away. Also watch for a swollen soft spot, if her pupils enlarge, if she seems lethargic (I know this is hard to tell in an infant that young), irritability, vomiting, or seizure. These things indicate that she needs to be seen immediately. Otherwise, it may take several weeks to go away, but it will eventually. Just let the doctor know that you're worried, and ask for information concerning how they plan to monitor it until it is gone. And congratulations to the new mom and grandmother!
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Lori