Katherine Heigl Introduces Son Joshua Bishop

Katherine Heigl might have chickens, donkeys, horses, dogs and two little girls at home in Utah, but she’s added one more to her bustling brood: Her first son, Joshua Bishop.

Heigl, 38, and her husband, musician Josh Kelley, 37, welcomed the 7 lbs. 15 oz. Joshua Jr. on Dec. 20.

“It was a surprise! I knew I wanted to expand our family, I knew I wanted to have more children,” she tells PEOPLE at her Utah ranch for this week’s cover story. “I just didn’t know how we were going to go about that.”

The actress calls herself “one of those people who really thinks, I have to schedule, I have to plan this out,” so finding out she was expecting just as she was back at work on her new CBS series, Doubt (premiering Feb. 15) wasn’t ideal.

“I’ll be totally honest, my first reaction was ‘Oh s—,’ ” she says. “But now we have a new normal, and I have to give myself a break in order to get used to it.”

Heigl didn’t always see pregnancy as part of her path to motherhood. For years, carrying a child herself “didn’t ever really speak to me. I got really invested in adoption. I didn’t look back or even think about it,” she says. “Only when I was about 35 did I start going, ‘Well, should we consider getting pregnant?’ You don’t want the choice to be taken away from you.”

Adoption has long been important to Heigl, whose close relationship with her older sister Meg, who was adopted from Korea three years before Heigl was born, helped shape her vision of her own family.

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

Joshua Jr. joins two older sisters Naleigh, 8, who was adopted from South Korea in 2009, and Adalaide, 4, who joined the family from Louisiana in 2012. He’s also got a third honorary big sister right now: Heigl’s niece, Meg’s daughter Madison, 15, currently lives with the family on the ranch to focus on her budding equestrian passion.

“This little boy is going to have all these girls fawning over him,” she says.

Now that she’s set to go back to work promoting Doubt next month, Heigl is chilling out in that fawning stage. “We’re at the point where we’re really obsessed with him!” she says. “With a newborn, you’re forced to be still … and you have this beautiful reason to do that.”

And even though she and her family are still adjusting to life with the little guy, Heigl already knows that she’s not done growing her family.

“I’d get pregnant again, and I still am very inspired by adoption. I also have been thinking a lot about fostering. It’s all up in the air,” she says with a smile. “And I’m okay with that!”

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.