The actor's wife Robyn filed for divorce last month after 28 years of marriage amid rumors that the actor had been dating 39-year-old Grigorieva.
The Braveheart star made their romance public weeks later, by taking his lover to a movie premiere in Los Angeles.
According to TMZ.com, Gibson has already told his estranged wife and their children that Grigorieva is in her second trimester of pregnancy.
The former couple share one daughter and six sons.
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