Bruni-Sarkozy says the pregnancy was "unexpected."
Asked to confirm rumors it's a boy, she told Friday's Nice Matin daily, "I don't even know, myself. There are things that are better kept secret."
The supermodel-turned-singer added she put plans to finish her third album on hold but she pledged the baby wouldn't derail her career.
Bruni-Sarkozy has a son from a previous relationship. Her husband, French President Nicolas Sarkozy, has three sons from two prior marriages.
The baby, which is due before presidential elections next year, is their first child together.