Jessica Simpson surprised by pregnancy

Jessica Simpson was "surprised" to learn she is

expecting her third child.

The 38-year-old actress and singer already has daughter Maxwell, six, and son Ace, five, with her husband Eric Johnson, and after revealing earlier this week she is expecting to expand their brood with another baby girl, sources say she was shocked to learn about her pregnancy.

An insider said: "Jessica and Eric are over the moon. They are so happy and very excited, surprised but overjoyed."

The Newlyweds: Nick and Jessica star isn't the only one excited about the new arrival , as her two young tots can't wait to be protective big siblings over their little sister.

The source added to Us Weekly magazine: "It's fun because Ace and Maxwell are at an age where they understand what it happening. They are really cute waiting for their baby sister."

Simpson's shock comes after she insisted in May last year that she wouldn't be having a third child, after reports surfaced claiming she was already expecting.

But the Blonde Ambition actress took to Instagram this week to share the happy news, when she posted a picture of her two children surrounded by pink balloons, which revealed the gender of the unborn baby.