Katherine Heigl Reveals the Emotional Way She Told Her Daughters She Was Pregnant

"I was so relieved to get to share it," Katherine Heigl tells PEOPLE of revealing the news to her daughters 12 weeks into her pregnancy

New baby, new life! Katherine Heigl, and her husband Josh Kelley, share the first photos of their baby boy Joshua Bishop Kelley Jr. Subscribe now for the exclusive photos and read why the mom of three left Hollywood – only in PEOPLE!

Katherine Heigl is working on a couple of big-time roles: she’s about to return to the small screen in Doubt and is now a new mom of three after welcoming her first son in December.

Her daughters Naleigh, 8, and Adalaide, 4, have new roles of their own, too: They’re now big sisters!

Sitting down at home in Utah for this week’s PEOPLE cover story, Heigl shares how she broke the news to her girls that they were getting a new baby brother, now 5-week-old Joshua Bishop.

Heigl and husband Josh Kelley waited until she was 12 weeks along before telling them that she was pregnant. “I was so relieved to get to share it,” she says, revealing her daughters were the first to hear about the baby after Kelley and Heigl’s mother, Nancy.

“[The news] was a little confusing for the girls because they’d be like, ‘I grew in your tummy?’ And I had to explain, ‘Well, you didn’t actually grow in my tummy, but your dad and I are your parents,’ ” she recalls.

Cheyenne Ellis

Because Heigl’s older sister Meg was adopted from Korea three years before Heigl was born, she is able to draw on childhood memories while raising her own family.

“My sister had a book growing up that we all read and we all loved called Why Was I Adopted?” Heigl says. “That was the first gift she gave Naleigh when Naleigh came into our lives.”

• For more on Heigl’s life with her new baby and her red-hot career, watch the full episode of People Cover Story: Katherine Heigl and Joshua Jr., available now on the People/Entertainment Weekly Network (PEN) and at People.com/heiglbaby. Go to PEOPLE.com/PEN, or download the app for Apple TV, Roku, Amazon Fire TV, Xumo, Chromecast, Xfinity, iOS and Android devices.

She also learned about an adoption-themed poem — “You didn’t grow under my heart, but in it” — which Heigl says helps instill the idea that “there’s no one way to be a parent.”

“My mother used to say this too: ‘It doesn’t matter how they come into your life,’ ” she says. ” ‘The moment they put that child into your arms — whether you just [gave birth] or whether they just arrived from Korea — they’re yours.’ ”

Heigl is aware that she may face people who don’t understand her family dynamic, but she says that Naleigh and Adalaide aren’t there yet.

“At the ages they are now, I don’t even think they much acknowledge how different we all look,” she says. “I don’t think that even factors into their consciousness at this point. It will as they get older, they’ll be like, ‘Wait a minute, we all look really different, and what does that mean for me?’ ”

Until then, “We’re there to support them as parents do. It’s all about open communication and helping them access whatever they need to feel absolutely 100 percent secure in who they are,” she says. “Whether that’s about their history, their heritage, their race, their memories … we’re there.”

For more from Katherine Heigl, pick up the latest issue of PEOPLE, on newsstands Friday.