The woman was pushing her child in a stroller on a trail between Kelly Avenue and Filbert Street after visiting friends at about 7:30 p.m., according to sheriff's officials.
She had just put her cellphone in her coat pocket when a suspect approached from behind and pushed her near Filbert Street.
The suspect and the victim both fell to the ground and the suspect began pulling at the woman's jacket, the sheriff's office said.
The child started to cry, causing the suspect to get up and run away.
The woman, who suffered a bruise on her knee, was not able to provide a description of the suspect. The child was not injured.
Anyone with information regarding the case is asked to call sheriff's Detective Jim Gilletti at (650) 363-4066 or the anonymous tip line at (800) 547-2700.