The Suddenly Susan star is due to give birth in May and already has a two-year-old daughter, Rowan.
She says, "I wasn't sure I was going to make this public. I'm thrilled for my daughter to have a little friend.
"Two Tauruses, it's a huge trick being played on me."
The 40-year-old star made headlines last year when she went public with her battle against postpartum depression after the birth of Rowan.
The actress engaged in a war of words with Tom Cruise, who publicly criticized the actress for championing anti-depressant medication.
In other baby news, Meg Ryan has returned home to Los Angeles after adopting a baby girl in China.
The When Harry Met Sally star flew to China to pick up her daughter over the weekend after spending months secretly fighting to get approval from adoption agencies.
The actress has a teenage son, Jack, by former husband Dennis Quaid.
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