The toddler is in critical condition at the Alberta Children's Hospital in Calgary. (D'Arcy Norman)

A Medicine Hat man has been charged with aggravated assault after a 21-month-old girl suffered a fractured skull earlier this week.

Melissa Hurkens, speaking on behalf of the child's mother on Friday, said the little girl is still in critical condition at the Alberta Children's Hospital in Calgary after suffering the injury Tuesday night.

"Mom was at work and the baby was being watched by mom's boyfriend. We aren't too sure yet how it happened. Officially I haven't heard anything," Hurkens said.

The boyfriend, who lived with the mother, took the child to the local hospital, said Hurkens. The girl was airlifted to Calgary and put on life support. Her mother and grandmother are at her side.

Sgt. Gord Stull of the Medicine Hat police said a 25-year-old man was arrested in Brooks and charged with aggravated assault in connection with the child's injury. He will be in court on Monday.

Hurkens said the mother is trying to hold it together, but she is "going through the motions."

"Because it's her own child [she] feels guilty, just because she thinks when you are a parent you are supposed to do everything you can and make sure they don't get hurt. We are all letting her know that it is not her fault and that we are all praying for her."

A trust fund has been set up for the family at ATB Financial in Medicine Hat.

"She is a single, working mom. She works hard to make sure her and her family are taken care of. So having to take time off work like this, for who knows how long — we don't want her to worry."

A court-ordered publication ban prevents the media from reporting the name of the child or her mother.