The 27-year-old actress – who's expecting her first child – wed longtime boyfriend Cash Warren, 31, on Monday in Beverly Hills.
"Jessica and Cash's ceremony surprised even good friends," says a source close to Alba, noting that the actress tends to be "very responsible" and "strategic" when it comes to her personal life.
"She [used to feel] it would be okay to wait until after the baby was born to get married," the source continues. "But her traditional side must have really kicked in."
The low-key wedding may have been a shock – but the two had been talking marriage for more than a year, says the source.
"Cash is a wonderful guy," the insider adds. "And she really does love him."
The two will welcome their daughter this summer.