Katharina Katit-Stäheli stabbed her son to death in a hospital in the Spanish coastal town of Torrevieja, local media reported on Thursday.
Despite being under police detention at a hospital in the southeastern Spanish coastal city of Torrevieja, she managed to trick officers to carry out the act, a Spanish police spokesman said.
"She cut his neck with a kitchen knife and then cut her own neck. Her prognosis is serious. The injury to her neck is serious," the spokesman said.
"She said she was going to bathe him or change his diaper. She took advantage of that moment to kill him," he added.
"The policeman with her saw her making strange movements and intervened. He saw she had cut the baby's neck and was trying to cut her own. It was all in the same movement. She managed to cut her own throat but his intervention prevented it going deeper.
She then required immediate surgery at the same hospital.
The 40-year-old had found herself in the hospital after being detained by police at Torrevieja's Carrefour shopping centre.
Her arrest came just days after Swiss authorities called on to Spanish police to help find the woman and her son Dylan.
Dylan had serious birth defects but Katit-Stäheli lost custody of him in December after refusing to take him to hospital for urgent medical treatment.
Katit-Stäheli then illegally took her son and the pair had been missing since Christmas.
On the Swiss woman's arrest, Dylan was found to be healthy but the two were taken to a hospital in Torrevieja so that Dylan could receive medical attention.
It is not known how the boy's mother obtained the knife or whether the two were alone in a hospital room when the alleged murder occurred.
Katit-Stäheli was described as a staunch vegan and horse lover by Spanish local daily La Verdad.
The paper also reported the woman has an adult daughter from a previous marriage, and that Katit-Stäheli has no contact with Dylan's father.