From History Channel:
It’s true that while many theories about the killer’s identity have emerged over the years, some of them more implausible than others (Lewis Carroll of “Alice in Wonderland” fame?), the police only had four actual suspects—all male. But after a witness said she saw the fifth Ripper victim, Mary Kelly, hours after she was murdered, the chief inspector in the case suggested it might have been the female killer escaping in Kelly’s clothing. Later proponents of this “Jill the Ripper” theory suggest that a midwife (possibly an abortionist) would have had the anatomical knowledge usually attributed to the Ripper, and would have had easy access to her female victims.
As the theory goes, the most likely suspect may be Mary Pearcey, who was convicted and hanged in 1890 for the murder of her lover’s wife and child—and who had used a method similar to the Ripper’s to commit the crime.