“THE HANGOVER” star JUSTIN BARTHA and new wife Lia Smith were the picture of joy during their beautiful garden wedding in her native Hawaii, but the lovebirds have another reason to be happy – she’s already pregnant!

Sources close to the couple say they learned the good news nearly five months ago but have been trying to keep it quiet.

“Justin and Lia were floored when they found out she was pregnant, but after they got over the initial shock, they were delighted,” a close source revealed. “After all, they were planning to start a family as soon as they tied the knot – they just got a head start.”

Justin, 35, and fitness-trainer Lia married on Jan. 4 at the Kualoa Ranch on the island of Oahu, a lush tropical paradise where the blockbuster movie “Jurassic Park” was filmed. They exchanged vows just before sunset in front of nearly 120 guests, including actress Reese Witherspoon and her husband Jim Toth, “Social Network” star Jesse Eisenberg, and Justin’s “Hangover” co-star Ken Jeong.

“Lia looked beautiful in an airy white dress that helped conceal her baby bulge, and Justin wore a black suit,” the source said. “Lia was a gorgeous glowing bride and Justin was almost tripping over himself to fawn all over her. They’re absolutely adorable together.”

After the ceremony, guests were shuttled to a reception, where they danced to both live music and a disc jockey and dined on fresh sea bass, prime rib, pastas and salads.

Justin, who previously dated Ashley Olsen, met his bride during a Pilates session at L.A.’s Equinox gym. They were first spotted together a year ago, celebrating the new year in Maui. The ENQUIRER broke the news of their engagement in our May 27 issue. In November, we shared exclusive details about their plans to marry in Hawaii.

According to the source, the baby is due in April.

“For now, they’re going to commute between Los Angeles and Hawaii,” the source added. “But they love the island life so much that they eventually want to move there to raise their family.”