A spokesman for the paper company said a worker was sorting through materials that were dropped off for recycling.
The facility takes in about 220 tons of material a day from 25 different municipalities, mostly from the San Gabriel Valley.
Homicide detectives from the L.A. County Sheriff's Department are working with Baldwin Park police on the case.
Police said the fetus appeared to be in its second trimester, but age, sex and cause of death has not been determined.
Anyone with information is urged to call the L.A. County Sheriff's Department at (323) 890-5500.
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