The actress, 38, is expecting her second baby with fiancé Joaquin Phoenix.
Mara attended the premiere of La Cocina at the Berlin Film Festival in Germany on Friday evening, when she cradled her growing baby bump under a long black dress.
Their son’s name honors Phoenix’s older brother, who died at age 23 on Oct. 31, 1993, from an overdose. He starred in films including Stand By Me, Running on Empty and My Own Private Idaho.
Mara and Phoenix’s baby news was revealed by Victor Kossakovsky, a Russian filmmaker who previously worked with Phoenix on the documentary Gunda, at the Zurich Film Festival. “He just got a baby, by the way,” Kossakovsky said in September 2020. “A beautiful son called River.”
Neither Mara nor Phoenix publicly acknowledged the pregnancy when the couple made rare red carpet appearances earlier that year, at the Golden Globes in January 2020 and the Oscars a month later.
Though Mara and Phoenix are very private about their relationship and role as parents, Mara did reveal that becoming a mother to River had “opened her heart” while celebrating her first Mother’s Day in May 2021.
Years later, while appearing on the LaunchLeft podcast, Mara spoke candidly about raising her son with Phoenix.
“It feels like a creative household, we’re constantly talking about creative things,” she said. “It doesn’t feel like a two-actor house because I didn’t work for the first three and a half years we were together. Basically, since we’ve been together, I only worked one or two times and one was really small and short.”
