Feb 26 (Reuters) - A California woman is expected to testify in court on Thursday about how using Meta Platforms' Instagram and Google's YouTube as a child harmed her mental health, as her landmark trial in Los Angeles continues.

The plaintiff, known as Kaley G.M. in court, began using Instagram at age 9 and YouTube at age 6 and says the platforms contributed to her depression and body dysmorphia. The companies sought to profit by hooking young children on their services despite knowing social media could harm their mental health, her lawyers say.

Kaley G.M.'s testimony comes a day after her former psychotherapist said adolescent social media usage was a "contributing factor" to her mental health issues.

The case is part of a broader global backlash against social media companies over alleged harms to children and teens. Australia has banned under-16s from such platforms, and other countries are considering similar restrictions.

YouTube and Meta have denied the allegations and said evidence in the case does not support the woman's claims.

The therapist was called by lawyers for the plaintiff to set the stage for the next phase of the trial, exploring whether and how Kaley's engagement with social media as a child affected her well-being.

The beginning of the trial focused on what the companies knew about how social media affects children, and their business strategies related to younger users. Meta CEO Mark Zuckerberg testified the company discussed but never launched products for children.

To win the case, Kaley's lawyers must show that the way the companies designed or operated the platforms was a substantial factor in causing or worsening her mental health issues.

(Reporting by Courtney Rozen in Washington, Jody Godoy in New York and Steve Gorman in Los AngelesEditing by Rod Nickel)

((chris.sanders@thomsonreuters.com))