Doctors were urging the former supermodel to have 'total rest' because she is an older mother giving birth in her 40s, France's Closer magazine reported.
The news of her pregnancy comes after reports this week that Bruni had 'fallen into depression' since the 57-year-old Sarkozy was defeated in last month's presidential election.
Friends said she had the 'baby blues' after giving birth to baby Guilia last September, and was now suffering from 'presidential blues' after quitting the Elysee Palace.
"For several weeks people have been asking why she had not been able to shift the extra weight for her last baby. But we can now reveal Carla Bruni-Sarkozy is pregnant again. And with her stomach looking ever more round, she can no longer hide it," the Daily Mail quoted the magazine as saying.
"She clearly wants to nurture this pregnancy with the utmost care, because, like the last one, it is later in life so all the riskier, and she has therefore been advised to take as much rest as possible," the magazine said.
"We wondered what the Sarkozy couple would do with themselves after leaving office, and now we know, play happy families!" it added.
Bruni, who had previous relationships with Mick Jagger and Eric Clapton also has 11-year-old son Aurelian with philosophy professor Raphael Enthoven.
Sarkozy has two sons, Pierre, 25, and Jean, 23, from his first marriage to Marie-Dominique Culioli, who he divorced in 1996.
He has a 13-year-old son Louis from his second marriage to Cecilia Ciganer-Albeniz, who he divorced in 2007.
Bruni and Sarkozy are currently living in her 3 million pounds Paris townhouse, where he has taken up a 10,000-a-month pound post on France Constitutional Council, the country's highest legal body, and she is planning to return to her career as a pop singer.