The subsequent investigation found "a gender reveal announcement resulted in the explosion" - and the woman was struck by a flying piece of debris, according to the press release.
A woman has been killed after an explosion at a gender reveal party went badly wrong.
Marion County Sheriff's office said in a statement: "Our investigation showed that members of the Kreimeyer family were experimenting with different types of explosive material on Friday and Saturday in an attempt to record a gender reveal that could be posted on social media for friends and family".
The metal piece struck Kreimeyer in the head, his office said.
Police said gunpowder and some coloured powder had been put inside a homemade stand which had a metal base.
Then, they put tape on top of the entire assembly, which "inadvertently created a pipe bomb", authorities said. The sheriff's office described her as a wife, mother and grandmother.
The woman was at a party for a couple to announce the gender of the baby they are expecting, sheriff's officials said.
According to news station KCCI, the death is being investigated by multiple agencies, including the Bureau of Alcohol, Tobacco and Firearms.
Find out about what exactly a gender reveal is.
"This is a reminder that anytime someone mixes these things there is a high potential for serious injury or death; please do not take these unnecessary risks".
Gender-reveal parties have become increasingly elaborate in the past few years, with expectant parents going to great lengths to ensure the moment of reveal is as elaborate as possible.