An expectant mother suffered a miscarriage after two people burst into a party to announce her baby's gender and opened fire.
The woman was shot in leg as the gunman sprayed bullets in the house in Colerain, Ohio. She lost the baby as a result.
Another 22-year-old woman was killed and three young children were injured, before the gunmen fled on foot, police said.
She was identified to local media as Autum Garret, from Huntington in Indiana.
"We're not sure if (the suspects) were allowed in or they forced entry at this point," said police Chief Mark Denney Denney. "We don't know who the suspects are and why they did this."
Partygoers were watching a film when the men started shooting, according to local media reports.
" T here's a whole lot of people here shot," one witness told a police dispatcher, in a frantic call.
He added: "I just seen two people walk by. I looked outside, next thing I know they just ran in and just started shooting."
Detectives from Cincinnati have opened a murder investigation into the incident. No arrests have yet been made.
A gender reveal party often involves the expectant parents and their gathered guests learning the soon-to-arrive baby's gender at the same time.
At some point a pink or blue is revealed by opening a box that's full of balloons in one of the colours.
