The British actors have been dating for over a year and debuted signs of their engagement -- Miller's dazzling diamond engagement ring -- in mid-February.
After remaining mum about her pregnancy for several months, the G.I. Joe: The Rise of the Cobra actress, 30, finally opened up to British Vogue in April.
"Even though I'm under strict instructions not to say anything as it would completely defeat the point of everything I've tried to achieve in the last eight years," the resolutely private Miller (who famously dated Jude Law) expressed to the mag. "I'm feeling fine...It's all progressing nicely and it's very exciting."