The attorney said Van der Sloot's Peruvian girlfriend, identified as Leidi, became pregnant during an unsupervised prison visit, DutchNews.nl reported Monday.
Van der Sloot, of the Netherlands, has twice been arrested in the 2005 disappearance of American Natalee Holloway. He also is wanted in the United States on extortion charges for reportedly having taken $250,000 from the teenager's mother for bogus information on the whereabouts of her daughter's body.
Peru has approved van der Sloot's extradition to the United States to face the charges. His U.S. trial is set to begin in 2015.
Van der Sloot was sentenced to 28 years in prison in Peru for the 2010 slaying of 21-year-old Peruvian Stephany Flores.
Holloway disappeared during a high school graduation trip to Aruba.