Dr David Webb, from the Philadelphia Department of Public Health and colleagues, examined the medical records of 37, 000 women who had given birth at more than 25 hospitals in the city between 1994 and 1997.

BBC: Daytime births 'more difficult'

Members of the public are being warned to be more vigilant about discarding personal records, such as credit cards, bank statements and utility bills.

BBC: Bin raiding: The growing scourge