The 30-year-old actress was last spotted with a bump late last month when she met new parents, Chris Hemsworth and Elsa Pataky, and their newborn daughter, India, reported Us magazine.
Other details such as the baby's gender, name and weight are not yet known.
The baby is Miller's first child with English actor Tom Sturridge. They had been dating over a year when the engagement was announced in mid February.
Her growing belly remained a mystery until she confirmed her pregnancy in an interview in April.