It happened inside the woman's home on 14500 block of Blue Sage Road in Adelanto late Wednesday afternoon.
A nearby neighbor reportedly called 911 after hearing the commotion.
"The female was taken for severe injuries believed to be caused by blunt force trauma. She was pregnant. Unfortunately, her unborn child was pronounced deceased at the hospital," said San Bernardino County Sheriff's Spokeswoman Jodi Miller.
Investigators say the 5-month-old fetus likely died as a result of the brutal beating.
Hines was immediately identified as the suspect after a witness told deputies they saw him running from the home.
Despite the violent beating that took place inside, neighbors say they were unaware anything was wrong until emergency vehicles arrived.
"I heard the ambulance and the fire truck and I heard the police, I saw them, they were taking pictures you could see the flashes until four in the morning," said neighbor Jennifer De La Rosa.
De La Rosa said she didn't know the couple well and only recently realized anyone lived in the home.
The victim remains in very critical condition with major head and upper-body injuries.
Meanwhile the manhunt continues for Hines, who is facing an attempted murder charge in the alleged assault on his girlfriend and a possible murder charge in the death of couple's 20-week-old fetus.
"Investigators don't believe that he is armed, however, he would be considered dangerous," said Miller.
Anyone with information on the whereabouts of Hines is asked to contact the San Bernardino County Sheriff's Department at (800) 783-7463.