A mother who was frustrated with her 23-month-old son slapped him so hard that he fell over and hit his head on a metal table leg.

This caused the boy to be so severely injured that he would from then on need help with his daily living needs.

Yesterday, the 27-year-old mother pleaded guilty to three charges of child abuse. She cannot be named to protect the identity of the child.

The court heard that the victim was the second of the woman's three sons. The older boy was four years old at the time of the incident in March last year, while the younger was aged one month.

Deputy Public Prosecutor Grace Chua said the woman frequently lost her temper with the victim, who she found to be slow in his development. The children's father did not live with them.

On March 25 last year, the mother was bathing her other two children at their home when she found the boy digging into his diaper and playing with his faeces, said DPP Chua. This angered her as she wanted to end bath time, and she slapped him hard once on the right cheek, said the DPP.

But the boy did not stop and the woman slapped him hard once more on the other cheek.

"The force of the accused's slap was so great that the victim's body turned midway and the victim fell sideways, with his head hitting hard against one of the legs of a nearby metal table," she said.

The boy cried and his body started to shake violently as his mother tried to calm him down. She massaged him with some oil, but he became barely responsive and was semi-conscious, the DPP added.

When the mother failed to wake him up, she called a friend for help as she was afraid all her children would be taken away from her if the authorities found out. But, at her friend's urging, she called an ambulance about 15 minutes later.

Several months later, the boy was found to have cerebral palsy as a result of his severe head injuries, and his developmental age was that of a six-month-old child.

He had to undergo surgery as his injuries left him unable to be fed orally. He was also assessed to be likely dependent for all his daily needs from then on.

Police investigations later revealed that this was not the first time the woman had assaulted the boy, said DPP Chua.

The woman had previously punched him more than five times in the chest when he did not listen to her instructions.

In another incident, she fractured his arm by yanking it forcefully when he refused to go to the toilet for a bath.

The woman will be back in court at a later date for her sentencing and mitigation.