(CNN) -- Every morning before school, nine-year-old Terisia Techu would undergo a painful procedure. Her mother would take a burning hot pestle straight out of a fire and use it to press her breasts.

With tears in her eyes as she recalls what it was like, Terisia tells CNN that one day the pestle was so hot, it burned her, leaving a mark. Now 18, she is still traumatized.

Her mother, Grace, denies the incident. But she proudly demonstrates the method she used on her daughter for several weeks, saying the goal was to make her less desirable to boys -- and stave off pregnancy.

A study found that one in four girls in Cameroon have been affected by the practice.

The U.S. State Department, in its 2010 human rights report on Cameroon, cited news reports and said breast ironing "victimized numerous girls in the country" and in some cases "resulted in burns, deformities, and psychological problems."

There are more than 200 ethnic groups in Cameroon with different norms and customs. Breast ironing is practiced by all of them.

Some mothers use hot stones or coconut shells to flatten their daughters' breasts.

Doctors believe improved diets have resulted in young Cameroonian girls going through puberty early. Many of them are also becoming pregnant early.

Terisia became pregnant at 15. Her child died at birth.

She told CNN that breast ironing doesn't work. She hates the practice and wishes her mother had instead talked to her about sex and preventing pregnancy.

Grace Techu argues that if it weren't for the breast ironing, Terisia would have become pregnant at an even younger age.

Techu has four daughters, and she used the procedure on the first two. The third avoided it because her breasts are growing at an acceptable rate, Techu says, and the fourth girl is still too young.

Mothers who want their children to finish school before becoming parents have resorted to this drastic measure, and many see nothing wrong with it.

In 2006, a German nongovernmental organization exposed the practice, which at the time was done mainly in secret.

Now, charities have embarked on campaigns to educate mothers in Cameroon that sex education -- not breast ironing -- is the solution to ending teenage pregnancy.

Dr Sinou Tchana, a gynecologist in Cameroon, has seen breast glands that were destroyed. She also saw one case of cancer, though she says it couldn't be established whether the ironing caused or only exacerbated the cancer.

"One mother came with secondary burns because the stone she was using to do this breast ironing burned her," Tchana says.

One of Tchana's patients is a 23-year-old whose scars are still painful 14 years after her breasts were ironed. She has joined the effort to confront mothers about the effects of their actions.

The challenge for all those trying to stop the practice is reaching parents like Techu in villages before a ritual that they say is motivated by love shatters more lives.

CNN's Josh Levs contributed to this report.