Swedish TV producer Anna Elisabet Eberstein gave birth to the 53-year-old film star's son last September in London, the magazine said.
The child's name was not reported.
Eberstein was pregnant at the same time as Tinglan Hong, who gave birth to her son Felix -- her second child with Grant -- last month.
UsMagazine.com said it is unclear what Grant's relationship status is with either woman.
Tabitha Xaoi Xi Hong Grant was born in September 2011.