|Robert Downey Jr and his wife Susan|
Nancy Miller told America's Star magazine: "I think it’s wonderful that she’s pregnant.
“The baby is due in February. Susan’s father, my brother, called me about three weeks ago with the good news. Her parents are over the moon about it.
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“It [doesn’t] really matter if it’s a boy or a girl. What really matters is that the baby is healthy.”
While the baby will be film producer Susan's first, Robert, 46, already has an 18-year-old son Indio from his first marriage to actress-and-singer Deborah Falconer.
Robert – who has battled drug addiction in the past - has previously credited Susan with providing him with a more stable relationship than he had with first wife although he admitted he is constantly surprised by how much work it takes to make a marriage successful.
He said: "Susan is cut from a slightly different cloth from Deb and I, and she's provided a great new framework for lots of things.
"Susan doesn't care if this building fell down and we never did another movie again so long as we were happy and loving and together, but so long as it hasn't, she's operating on the fact there's a lot of s**t to get done.
"She is a master at it - I wouldn't be surprised if she built the pyramids.
"Do we have to work at the relationship? More than I thought. And usually more than I would care to.
"But ultimately, it's nice to have pure communication with, for me, a woman. Historically this is not something that, for me, was not really achievable."