She left when a psychiatrist for the defense testified that Mitchell had been motivated to kidnap Smart because of his desire to have children and create a new race.
Forensic psychiatrist Paul Whitehead, who has examined Mitchell, said he gathered this information from the journals of Wanda Barzee, Mitchell's estranged wife, according to local KSL-TV reports. Whitehead has testified that he believes Mitchell suffers from a "major psychotic illness" and a personality disorder.
On Wednesday, the doctor said that Mitchell chastised Smart for not wanting to have children after he and Barzee got baby clothes for the child. The clothes had been stolen.
Whitehead testified that Smart had a name picked out for the baby, say news reports. After this statement, a visibly upset Smart looked sternly at Whitehead, then stormed out of the courtroom, followed by her mother, Lois.
Neither returned to the courtroom after the judge called a recess.
Smart's abrupt exit marks the second day of drama in the federal kidnapping trial of Mitchell, 57. On Tuesday, he apparently suffered a seizure and had to be taken by ambulance to the hospital. He was released shortly afterward and returned to the Salt Lake County Jail.
Wednesday morning, showing no ill effects from the previous day's events, he entered the courtroom singing "O Little Town of Bethlehem" and "Hark, the Herald Angels Sing" before he was removed from court.