The 41-year-old actor - who already has seven-year-old son Jack with his ex-wife Anna Faris - and the 30-year-old author were believed to have welcomed their first child together when Katherine's brother, Patrick Schwarzenegger, revealed the news this week.
They kept her pregnancy relatively quietKatherine Schwarzenegger and Chris Pratt attend the premiere of Walt Disney Studios Motion Pictures "Avengers: Endgame" on April 22, 2019 in Los Angeles, California.
The couple were first seen together in June 2018, nearly a year after Pratt and Faris made a decision to "legally separate".
On Instagram Live in June, Schwarzenegger discussed her experience being pregnant during the coronavirus (COVID-19) pandemic.
Chris Pratt has revealed the name of his and Katherine Schwarzenegger's daughter.
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"We couldn't be happier", Pratt wrote. Both mom and baby are doing great. "We are extremely blessed", the action star added, followed by Bible verses. "Love Katherine and Chris".
"They're doing great - just got her a little gift", Patrick said in footage obtained by ET on August 9.
Katherine Schwarzenegger is the elder daughter of actor Arnold Schwarzenegger and journalist Maria Shriver.
This is fitting, as Shriver, a member of the Kennedy family, was the one who set them up. The source continued, "She is going through all the first-time mom emotions".