"Maxwell and Ace cannot wait" for their new baby sister, a source tells PEOPLE, adding that Jessica Simpson's pregnancy "was a very happy surprise"

By Jen JuneauLiz McNeil and Aili Nahas
September 18, 2018 01:35 PM

Jessica Simpson is positively glowing.

The pregnant singer, 38, shared the first glimpse of her baby bump to social media Tuesday morning, hours after revealing that she’s expecting her third child with husband Eric Johnson.

Simpson dons a long multicolored dress featuring bell sleeves and black panels in the photo on Twitter and Instagram, wearing a pair of sunglasses and her long tresses over her shoulders as she cradles her belly.

“My Baby Love,” she captioned the outdoor photograph.

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Simpson announced she’s expecting again with a photo set of her two kids, Ace Knute, 5, and Maxwell “Maxi” Drew, 6. She also shared that she’s having a girl.

“This little baby girl will make us a family of five,” she wrote alongside a snapshot of Ace and Maxi surrounded by pink balloons. “We couldn’t be happier to announce this precious blessing of life.”

A source tells PEOPLE exclusively of the family’s reaction to the baby news, “They are SO excited. Maxwell and Ace cannot wait — and it was a very happy surprise.”

A surprise — but a welcome one.

“Jessica and Eric absolutely thought they were done having kids after Ace was born — one of each was perfect,” a second source tells PEOPLE. “She went through a lot during and after her pregnancies. But that obviously wasn’t meant to be! Jess is such a great mom — Maxi is like her mini-me. Eric is the best dad too — there is a lot of laughter in that house.”

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Last month, Simpson surprised the crowd at the Orange County Fair, joining close pal and Dukes of Hazzard costar Willie Nelson during his concert set for her first onstage performance in years.

She hid her baby bump on stage in a black-and-white polka-dot dress and made-for-walking bedazzled, knee-high cowboy boots, sharing an embrace with Nelson, 85, before launching into a duet with the icon.

“It’s not every night that a legend invites you to join him on his stage,” Simpson — who memorably dressed up as Nelson for Halloween in 2017 — captioned an Instagram post from the concert. “And my husband and kids got to see me perform for the first time! I love you @willienelsonofficial.”