Dominique Cottrez, 51, appears in the courtroom of Douai, northern France, accused of multiple counts of first-degree murder of minors. (AP)

A French woman accused of killing eight of her newborns over more than a decade has been sentenced to nine years in prison.

After deliberating a few hours in the northern city of Douai, the jury returned its verdict - half as long as the 18-year prison term asked by the prosecutor against Dominique Cottrez, a 51-year-old married mother.

The jury decided that her judgment was impaired at the time of the killings.

They convicted Cottrez of seven counts of first degree murders and one count of second degree murder for the first baby she killed in late 1989. The last infant was killed in 2000.

The new owner of the Cottrez family's house, in a village in northern France, discovered the dead babies in rubbish bags buried in the garden or hidden in the garage.

Cottrez risked life in prison. But prosecutor Eric Vaillant in his closing arguments on Wednesday asked the court to grant the defendant "extenuating circumstances" because he said she suffered from psychological problems and from proven neuroses.

Her two living daughters - Emeline, 28, and Virginie, 27 - implored the court not to send their mother back to jail.

Cottrez has already spent two years in prison for temporary detention at the beginning of the investigation, and was then was released from jail in 2012. The mother, who suffers from hyper-obesity as well, was expected to spend the next night in prison.

For more than four years, Cottrez convinced police investigators, psychiatric experts, the investigating judge and lawyers of a false story - that she had been raped by her father as a child and was then involved with him in a long, incestuous relationship as a married woman.

In court, she initially said she felt compelled to kill her babies because she feared they were the product of incest.

But she confessed to the court during Monday's hearing that she had not been raped by her father, her lawyers said.