why does pip think that molly is estella's mother?

The Jaggers invites Pip to dinner, where he gives the young man a note from Miss Havisham. When Jaggers mentions Estella’s marriage shortly after Jaggers’s housekeeper Molly walks in, Pip realizes that Molly is the person he couldn’t place, the person Estella mysteriously resembles. He realizes at once that Molly must be Estella’s mother.