The corpses were discovered in individual plastic bags hidden around two houses in the tiny village of Villers-au-Tertre, about 35 miles from Lille.
Police were alerted after a family moved into their new home and found infant bones in the garden.
Officers traced the previous occupants, who were living less than a mile away in the hamlet.
Sniffer dogs found the bodies of more new-born babies there.
"Two people are being held in custody. They are the mother and father of the children," said public prosecutor Eric Vaillant.
Newspaper reports in France said the unnamed couple are both in their 40s.
Le Figaro said two of the bodies had been hidden in the first house for more than 20 years.
France has seen a string of cases in recent years of mothers killing their new-born babies and hiding the corpses.
In March, a 38-year-old mother was jailed for 15 years for killing six of her newborn babies and hiding their bodies in bin liners in the cellar of her home in Valognes, near Cherbourg.
Céline Lesage was found guilty of suffocating four babies and strangling two others after giving birth to them in secret between 2000 and 2007.
Another French woman, Veronique Courjault, was convicted last year of murdering three of her new-born children.
Her husband discovered two of the corpses in a freezer while the two were living in South Korea.
During the trial, psychiatrists said she suffered from a condition known as "pregnancy denial."
I think this is one of the most shocking story's I have ever heard.
Bodies Of Eight New-Born Babies Found - Yahoo! News UK
French couple held after 8 dead babies found - Yahoo! News UK
Their inquiries led them to the couple's home in the village around a kilometre (less than a mile) away, where six more babies' bodies were found, the councillor said.
A judicial source said the case could turn out to be the deadliest infanticide incident ever known in France.
The detained couple were respected in the community, the councillor said. The husband worked in the building trade while his wife worked as a nursing assistant.
"I'm still in shock," the former mayor of the village, Daniel Collignon, told AFP, describing the village as a very calm and rural place.
Neighbours also reacted with astonishment to the news.
"They are normal people, who even have a role in the community," said one. "It's incredible."
Another neighbour, a man in his 50s, added: "These are attractive, helpful, polite and courteous people."
They had done nothing to suggest that they might be capable of abnormal behaviour, he said.
"The husband was even elected to the town council," he added.