The actress is expecting a baby with fellow actor and boyfriend Tom Sturridge, according to Miller's stepmother, Kelly Hoppen.
Hoppen told the Evening Standard: "Sienna is definitely not engaged or getting married, and she is pregnant."
Miller and Sturridge were first reported to be expecting a child together at the start of the month. Neither of the couple's representatives has commented on the news.
Miller and her sister Savannah recently stepped down as co-creative directors of their fashion label Twenty8Twelve to focus on new ventures.