Lela Tabidze, Mariam Makhniashvili's mother, told the CBC's Mark Kelley her family will mark the girl's 18th birthday on Tuesday by spending the day together. (CBC)

The mother of Mariam Makhniashvili, a Toronto teen who went missing more than a month ago, says her family still doesn't believe the girl left home of her own accord.

In an interview with the CBC's Mark Kelley, Lela Tabidze said "every minute we're sitting close to the telephone and we grab it whenever it calls.

"The pain we have right now, it's the ultimate pain because we know nothing. We're helpless," she said.

Mariam, whose 18th birthday is Tuesday, has not been seen since she walked with her younger brother, George, to Forest Hill Collegiate Institute on the morning of Sept. 14.

She and George separated at the back entrance to the school, with Mariam deciding to use the front entrance.

Police have been unable to say with any authority what happened to her after that.

"So what we do when we're alone, my husband especially, he goes searching statistics, what kind of facts we have, like sometimes how people disappear," Tabidze said.

She said she searches her memories of Mariam for "clues [as to] what might have happened, how she would behave in the case if somebody would approach her."

"Or if she somehow — we don't believe it — or if she somehow decided to leave."

Mariam Makhniashvili was last seen Sept. 14. (Toronto Police Service)

No bad news yet

Tabidze said they will mark Mariam's birthday, even though "it won't be the normal celebration, of course."

"But we would be together, still, [to] support each other and still search … for some hope."

She said the fact that they had no negative news was their one cause for optimism.

Mariam and her brother lived in Tblisi, Georgia, before moving to Toronto in June to reunite with Tabidze and their father, Vakhtang Makhniashvili.

The parents arrived in Canada from Los Angeles a few months earlier. The two had lived in the U.S. for five years.

"We were so safe. Everything here for Mariam was perfect," Tabidze said.

Police said to the best of their knowledge, she had no friends in the city, and her only relatives in Canada are her parents and her brother.