In a statement, a representative for the Sex And The City star and her actor husband confirmed that the couple were "overjoyed" by the news.
The spokesperson added that the family was "happily anticipating the birth of their twin daughters later this summer with the generous help of a surrogate".
Speaking to Entertainment Weekly, an unnamed friend of the pair said: "They're over the moon and excited as any prospective parents would be. Their life is about to get a lot busier."
Further details of the pregnancy and the identity of the surrogate mother have not been revealed.
The couple, who married in 1997, are already parents to six-year-old son James.