Actor Joaquin Phoenix and wife-to-be Rooney Mara confirmed they welcomed a son into the world.
They kept the birth quiet for two months.
But now Joaquin, 46, and fiancée Rooney, 35, have spoken about their new arrival for the first time publicly.
Russian filmmaker Victor Kossakovsky accidentally let slip last month but until now the couple have decided to stay silent.
The stars have named their son after Joaquin's brother River who tragically died in 1993 from a drug overdose.
They used their first chance to speak about being new parents to discuss the migrant child crisis.
The actors and activists spoke to People about nearly 550 children who have yet to be reunited with their parents having been separated at the US-Mexico border.
In a lengthy joint message, the passionate pair drew attention to the situation after the Joker actor confirmed their baby news.
They lamented the thought of being separated from their child was simply "unbearable" as new parents.
The couple said: "Last week, we learned that the parents of 545 children separated at the border by immigration officers have not yet been found.
"The weight of that number is staggering. Five hundred forty-five children...
"As new parents, it's unbearable to imagine what it would feel like to have our child taken away from us for a day, let alone years.
"But that's the very situation those 545 children and their parents have been living through."
