Relatives of a woman who hid her baby in the maggot-infested boot of a car have said they had no idea of her secret, during a trial that has drawn horror in France.

Rosa-Maria Da Cruz, 50, is accused of keeping her daughter Séréna hidden away until she was nearly two, leaving her with serious learning disabilities.

Séréna was discovered in 2013 in the filthy boot of a Peugeot 307 and had also been kept in an unused room at the family home in the Corrèze region of central France.

It was a mechanic who found her in the boot after Da Cruz, who is originally from Portugal, had taken the car to be repaired. Hearing a noise, he opened the boot to discover the baby in a car seat, naked, filthy and dehydrated. She was surrounded by maggots and excrement.

The trial, which opened on Monday, has heard testimony from relatives about how Da Cruz was, to all appearances, a model mother to her three other children.

Her partner, Domingos Sampaio Alves, has said he had no idea she had given birth to another child and she had kept it a secret. By way of explanation, he said he went only about once a month to the small ground-floor room, under renovation, where Da Cruz kept the baby much of the time. He also did not visit the car often as he does not have a driving licence.

“I don’t know why she did this,” he told the court in Portuguese through a translator.

The unemployed bricklayer was originally charged, but investigators concluded there was no reason to doubt his account. Da Cruz was released pending trial, and the couple have stayed together. “I continue to live with her because she is a good mother to the children,” Alves told the court.

They have kept their three older children – aged nine, 14 and 15 – but authorities have refused to let them have contact with Serena, who is now aged seven and living with foster parents.

In her opening testimony on Monday, Da Cruz fought back tears and told the court: “It’s very hard to be confronted with reality, with the damage I have done.”

Séréna now suffers from severe mental impairments including irreversible autism, which medical experts have linked to sensory deprivation during her early months.

A paediatrician who examined Séréna and her siblings after the discovery said her team had been “stupefied” by “the gulf between Séréna’s situation and those of her brothers and sisters”, who appeared to have been “perfectly raised”.

“You could see she had been a good mother, and we could not understand why Séréna had not received the same quality of care at home,” the doctor said.

The judge Gilles Fonrouge said that while the child was now in good physical health, she was “closed off to interactions around her”.

Marie-Pierre Peis-Hitier, a lawyer for the social services, said the child could say only a handful of words, and it had taken a full afternoon to win the child’s trust.

Da Cruz’s sister and nieces also said they had no idea of Séréna’s existence, and said Da Cruz was a “loving” mother to the other children.

“Inside I think she is suffering. She knows what she has done,” her niece Elodie told the court.

The court heard that Da Cruz had initially hidden the pregnancies of two of her other children from her partner, not wanting to face reality.

Da Cruz is charged with violence against a minor and could face up to 20 years in prison if convicted. The trial continues.