James Holmes (2nd L) stands at the start of his trial in Arapahoe County District Court in Centennial, Colorado in this still image captured from a pool video footage April 27, 2015. Reuters

CENTENNIAL, Colo. (Reuters) - A woman who survived Colorado's movie theater massacre found an escape route blocked by bodies, she told jurors on Tuesday in the murder trial of gunman James Holmes. And a nine-months pregnant woman slipped in blood while fleeing with her badly wounded husband.

Survivors of the mass shooting told of their terror for the first time in court as the gunman looked on. Twelve people were killed and 70 were wounded in July 2012 when Holmes opened fire inside the crowded theater in the Denver suburb of Aurora.

Holmes, a 27-year-old former neuroscience graduate student, has pleaded not guilty by reason of insanity to multiple charges of murder and attempted murder in the rampage at the theater, which was showing the Batman movie "The Dark Knight Rises."

His long-awaited trial began in Arapahoe County District Court in nearby Centennial on Monday. The 19 women and five men of the jury were asked to decide whether Holmes was insane when he plotted and carried out the attack, or a calculating mass murderer who deserves to be put to death.

On Tuesday, the prosecution's first witness, Katie Medley, described how she was nine months pregnant when she went to see the movie with her husband, Caleb, an aspiring stand-up comedian.

Her husband, who was shot through the eye, testified briefly from his wheelchair, just a few feet away from Holmes, who looked on expressionless.

The brain injury Caleb Medley suffered left him unable to walk or speak clearly. Instead, he pointed at an alphabet board to spell out his answers as he confirmed his name.

After the gunfire began, his wife said she quickly realized he was badly hurt: "I saw blood pouring from his face, and I knew he got shot in the head," she told the hushed courtroom.

"I told him that I loved him, and that I would take care of our baby if he didn't make it."

Two days later, she gave birth to their son Hugo in the same hospital where her husband was undergoing multiple surgeries.

Yelling, Screams

Another survivor, Munirih Gravelly, an employee of Aurora's Buckley Air Force Base, went to the Century 16 multiplex with a friend who was killed there.

She said the movie had been playing for about 15 or 20 minutes when a tear gas canister was thrown into theater nine, and then the shooting started. She lay on the floor.

"I heard all the gunfire and a lot of yelling, you know, out of confusion, but then it shortly turned to screams. I heard a lot of people calling other people's names," she said.

During a pause in the shooting, she tried to crawl to safety but froze when the shots began again.

Finally the gunfire and the movie stopped, and the lights came on.

"Then I could see down the aisle I was in, and I wouldn't have been able to get out anyway because there were bodies in it," Gravelly said. She had to step over her friend's corpse to leave.

None of the prosecution witnesses were asked any questions by Holmes' lawyers.

Aurora police Sergeant Michael Hawkins said he was walking to his car after his shift when he heard the "shots fired" call.

Speeding to the scene, he listened to reports of tear gas and multiple gunshot victims. In theater nine, he found a man suffering a terrible wound.

"Most of his head was gone," Hawkins told jurors.

Fighting back tears, he then described carrying the youngest victim, Veronica Moser-Sullivan, 6, to an ambulance. She had been shot multiple times and later died.

On Monday, prosecutors said Holmes, who was armed with a handgun, shotgun and semiautomatic rifle, carried out the massacre because he had lost his career, his girlfriend and his purpose in life, and had done it "to make himself feel better."

In their opening statement, Holmes' public defenders said he was suffering from schizophrenia. He had long heard voices in his head commanding him to kill, and he was not in control of his actions "or what he perceived to be reality."

The trial is expected to last four or five months. Prosecutors have said they will seek the death penalty if Holmes is convicted.