More than 200 policemen have joined the search for the missing newborn, which now covers a 10-mile radius.
Ms Marten, the daughter of British aristocrat Napier Marten, and Mr Gordon were on the run for seven weeks, having disappeared following the birth of their child in January.
Neither the mother nor child are believed to have seen a doctor since the birth, and the couple has gone to great lengths to avoid capture over the last month.
They reportedly paid only in cash, disguised themselves in front of CCTV, and may have booked hotels under pseudonyms.
