The attack happened around 8 a.m. near Cienega and Valley Center avenues in the small community just west of San Dimas. Authorities said the woman was walking when she was approached by a man who knocked her to the ground and began to kick her.
According to investigators, the woman screamed that she was pregnant and the suspect immediately fled the area.
The woman was taken to the hospital to be treated. Her condition was not known.
Investigators said they don't know why the victim was targeted. She said she did not know her assailant.
The suspect was described as a 23- to 25-year-old Hispanic man, about 5-foot-5 to 5-foot-8, weighing around 160 pounds. He was wearing a black sweatshirt and sunglasses.
The attack has residents in the area concerned.
"I just think it's getting really bad now. You can't even walk around and feel safe anymore," said San Dimas resident Mary Helen Ramirez.
Anyone with information on the case was asked to call the Los Angeles County Sheriff's Department San Dimas station at (909) 450-2700.