Pamela Kreimeyer died instantly when metal debris struck her head, the Marion County Sheriff's Office said on Monday, according to ABC.
The debris then flew another 132m before landing in a field in rural Knoxville, 35 miles southeast of Des Moines, police said.
It was reported that the 56-year-old was 14 metres from the device when it exploded.
The family had been experimenting with explosives for a colourful announcement of the baby's gender on social media, authorities added.
They packed a metal cylinder with gunpowder that they thought would discharge the coloured baby powder.
But authorities say tape covering the top of the cylinder caused it to detonate like a pipe bomb.
"This family got together for what they thought was going to be a happy event with no intent for anyone to get hurt," Sheriff Jason Sandholdt said in a statement, ABC reported.
"What ended up happening was that Pamela Kreimeyer, a wife, mother and grandmother, was killed by a piece of metal where a metal stand gunpowder and coloured powder were involved," he added.
Parties where parents reveal the gender of their unborn child to friends and relatives with some kind of stunt have become increasingly popular in the last decade, particularly in the US.
The Standard have contacted the Marion County Sheriff's Office for comment.
