Why would a nurse call 911 in a hospital?
She probably is mandated to report that no fetus was found with the woman when she came in because the fetus would have been sent to autopsy to determine what happened to it and to rule out the mother having done something to kill a live fetus. It’s probably just SOP.
Even if an upper class woman goes to the ER for bleeding and has had a history of prenatal care and she miscarried right then and there in the ER because of a pregnancy tragedy that just sometimes happens they’d do basic tests on the fetus. They’d be legally bound to rule out heavy drinking or drugs versus the miscarriage resulting from some freak congenital thing that just sometimes happens. They’d also want to see if there is some sort of condition the woman needs to know about to help make a future pregnancy not end the same way.