Authorities said driver fatigue may have contributed to the crash. The previous night, the driver had traveled from Fresno to Los Angeles, then left Los Angeles shortly after 3 a.m. Sunday. He had been on the road for about four hours when the bus overturned.
Dozens of passengers among the 44 people aboard the San Francisco-bound bus were hurt, at least seven of them with major injuries.
Four survivors were trapped in the wreckage and had to be rescued with hydraulic equipment, while some of the most seriously injured were airlifted to hospitals, authorities said.
Faro Jahani, 50, of San Francisco, and Martha Contreras, a 23-year-old Santa Maria resident who was seven months pregnant, were killed, said Lt. Dan Minor of the California Highway Patrol.
Seven other people suffered major injuries, four had moderate injuries and 31 had minor injuries after the bus went down an embankment along Highway 101 in Santa Maria shortly after 7 a.m., said Santa Barbara County Fire Capt. Keith Cullom.
Minor said a preliminary investigation gave no indication of mechanical problems, and the bus driver didn't appear to have been impaired by alcohol or drugs.
"We do have reason to believe that driver fatigue may have been a significant factor," Minor said.
The bus drifted off the freeway about three miles from its intended off-ramp and came to rest on its right side a few feet down an embankment after striking a eucalyptus tree.
Both northbound lanes of Highway 101, one of the state's major corridors, were shut down after the accident and remained closed until mid-afternoon as the California Highway Patrol investigated. The closure caused a backup that stretched for two miles, officials said.
Santa Maria, which was in the media spotlight this year during the four-month child molestation trial of singer Michael Jackson, is about 75 miles north of Santa Barbara.
Three buses were sent to Santa Maria to pick up passengers able to continue the trip, said Kim Plaskett, a spokeswoman for the Dallas-based bus line.