The couple from the southern town of Montpellier, aged 23 and 35 respectively, were initially arrested on charges of condoning terrorism, a judicial source said.
But national counter-terrorist prosecutors were brought in last week after investigators found the homemade fake stomach, which could have been used to hide objects.
No explosives were found but an analysis of their computer found they had consulted bomb-making information and photos of jihadists, a source close to the investigation said.
The couple have not yet been charged.
Their arrest comes with France on high alert following the attacks in Paris of November 13 that left 130 dead.
Police say they foiled a jihadist plot last week near Orleans southwest of Paris by two men who were planning to attack "soldiers, police and representatives of the state".
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