Yesenia Sesmas, from Texas, was found guilty on Tuesday of shooting Laura Abarca in Kansas and kidnapping the victim’s newborn baby.
The 36-year-old intended to raise the child as her own after she lost her own child months before, prosecutors said.
She had pretended to friends and family that she was pregnant again – holding a baby shower and preparing her home for a new arrival.
Sesmas discovered that Ms Abarca, who she worked with in a restaurant years ago, had given birth – and drove to her home in Wichita, Kansas.
She shot the 27-year-old in the head before abducting six-day-old Sophia Gonzales in the November 2016 attack.
The baby was found safe less than two days later at Sesmas’ apartment in Dallas and was returned to her family.
The jury found Sesmas guilty of first-degree premeditated and intentional murder, kidnapping and interference with parental custody.
She faces life in prison when she is sentenced in July.
Sedgwick County District Attorney Marc Bennett told jurors that after Sesmas had faked a pregnancy, prepared a home for a new baby and had a baby shower, she “was either going to have to come home with a baby” or explain that she had lied to those who knew her.
Sesmas went back to Texas thinking “she would wake up and raise that baby and her life would begin”, Mr Bennett said.
In interviews with investigators and the media, Sesmas said Ms Abarca had agreed to give her the newborn but changed her mind after the birth.
She said she had not intended to kill Ms Abarca and had only brought the gun to intimidate her.
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“She didn’t know that it (the gun) was going to fire and she was surprised when it did,” public defender Jason Smartt said.
Jurors deliberated for less than three hours before reaching their verdict.