A couple who adopted a baby boy five years ago discovered that his biological mother had another child — a girl, this time — so they adopted her too.

What are the details?

Joseph and Shanna Weight of Elk Ridge, Utah, adopted 5-year-old Milo from his birth mother when he was just a baby.

According to ABC News, Milo's birth mother contacted the Weights in January and told them that she was pregnant again, and proposed that the Weights adopt her unborn daughter.

They were glad to acquiesce, and adopted Onni, who is now 3 months old. The children's biological mother was just 18 years old when she adopted Milo out to the Weights.

"Milo, he was kind of the perfect first child," Shanna Weight told "Good Morning America" about her children. The couple also has their own biological child, 2-year-old Nash. "[Milo] was our entire world. I can't explain what he's done for our lives. Nash's exuberance and fun keeps on our toes. Onni has a sweet calmness to her. She's just pleasant and happy, but particular. She makes sure you get it right."

Joseph Weight added, "[The children's birth mother] was a young mom and she had grown up with a lot of siblings from different fathers. She never knew her dad, so she wanted her child to have something better than what she had."

The adoption agency that they'd used with Milo contacted the family once again at the birth mother's request to gauge their interest in adopting her daughter.

"[The organization] contacted us and said, 'Milo is going to be a big brother' and asked if we would consider adopting his sister," Joseph said. "At the time, I was like, 'That's a big one.'"

Shanna said she initially balked at the idea.

"My first thought was, 'There's no way we can do this. We're not ready,'" she said, adding, "After the initial shock wore off I thought, 'It would be so nice to have them raised together.'"

Joseph said he gave the news to Milo that the family would be bringing his biological sibling into the Weight family. Needless to say, Milo was very excited by the news.

"I [told Milo], 'Your birth mom is pregnant!' and he started jumping up and down," Joseph recalled. "He pointed his finger right in my face and said, 'We're bringing home that baby.'"

Shanna said she felt nothing but gratitude for being able to bring Onni into the family.

"I felt very grateful that she would give us the opportunity to also parent one of [Milo's] siblings," Shanna said. "I think how courageous [their birth mom] was to recognize that they needed something more than she could give them. She was very selfless in what she did."

"I'm thankful every single day that she chose us and felt like we'd be good parents," she added.

As for Onni, she seems to be settling in quite nicely.

"She has a dimple just like Milo," Joseph said. "He loves to talk to her and make her smile. She's a smiler."