Shields, 40, and husband Chris Henchy, a 42-year-old television writer and producer, also have a daughter, Rowan, who turns 3 next month. They have been married since 2001.
The birth came the same day that Tom Cruise and Katie Holmes welcomed the arrival of daughter Suri.
Shields and Cruise had a public spat last year after the "Mission: Impossible" star criticized the actress for taking antidepressants after the birth of her first child.
Appearing on NBC's "Today" show, Cruise said there was no such thing as chemical imbalances that needed to be corrected with drugs, and that depression could be treated with exercise and vitamins.
Shields called the remarks "a disservice to mothers everywhere" and said the drugs helped her survive feelings of hopelessness after the birth in 2003.
"I'm going to take a wild guess and say that Mr. Cruise has never suffered from postpartum depression," she wrote in an op-ed piece in The New York Times last July.
Shields said that at the lowest point of her depression she considered swallowing pills or jumping out a window. A doctor attributed her feelings to a plunge in estrogen and progesterone levels and prescribed Paxil.
Shields chronicled her experiences in "Down Came the Rain: My Journey Through Postpartum Depression."
Shields, who began modeling as a toddler, has appeared in films including "Pretty Baby," "The Blue Lagoon" and "Endless Love." Her TV sitcom, "Suddenly Susan," ran from 1996-2000.