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Katherine Heigl and Josh Kelley show off their adorable baby boy


UPDATED MARCH 1, 2019 7:50 AM EST

Katherine Heigl's adopted daughters Naleigh Moon and Adalaide Marie Hope “were excited” to become big sisters, but were initially confused that their mom had to grow their baby brother first. The actress, who recently welcomed her first son Joshua Bishop Kelley Jr. with her husband Josh Kelley, opened up about carrying her first child instead ofadopting like her other kids.

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“The girls were excited to discover they were going to have a babybrother, but very confused when I explained that I was pregnant and had to growhim first!!!,” the mom-of-three revealed in an Instagram post on Thursday.

KATHERINE HEIGL AND JOSH KELLEY'S PRECIOUS FAMILY MOMENTS

Katherine and Joshadopted their two daughters in 2009 and 2012 respectively. Chatting to People magazine about experiencing her first pregnancy, the Doubt star confessed,“I actually prefer the adoption way because I wasn't subject to hormones.”

She explained, “One minute you’re weirdlyobsessed with this baby, like ‘Don’t take him out of my sight,’ and the nextyou’re kind of blue, you’re a little sad and a little freaked out.”

Photo: Instagram/joshbkelley

However the hormones were admittedly “the onlydifference” when it comes to her children. “The moment you hold your child, you’re whollyin love,” she said. “And you’ll do anything to protect them and shelter them.”

A look at Katherine Heigl's 'Pinterest-worthy' baby shower

Katherine, who announced she was pregnant last June,gave birth to her first biological child on December 20. In her candidinterview, the 38-year-old admitted, “I’venever been determined to experience pregnancy.”

She continued, “But I’m so grateful that I did. Ithink that if it hadn’t been a surprise, I’m not sure I would have done it. Itwas an incredible experience, it was kind of magical.”