The 34-year-old rapper also shared how he's balancing family, a new album and life on the road.
Macklemore didn't tell his daughter that she'll be a big sister -- she told him!
ET's Katie Krause spoke with the rapper at CBS Radio's We Can Survive concert in support of Breast Cancer Awareness Month on Saturday, where he revealed the "emotional" way he found out his wife was expecting baby No. 2.
"My wife gave [Sloane] the pregnancy test, put it in a fanny pack and said, 'Sloane has a present for you,' and she opened it, gave me the pregnancy test," Macklemore revealed. "She broke [the news] to me!"
"It was pretty crazy," he added. "I definitely got emotional and I was just in pure shock."
Macklemore has been in a house full of girls since his wife gave birth to Sloane in 2015, and he says his daughter is hoping it stays that way.
"She really wants a sister. If we have a girl, I think she's going to be pumped. If we have a boy, we'll see... We're going to wait [to find out the sex of the baby]," he said, before dishing on Sloane's Halloween plans.
"She is going to be Belle from Beauty and the Beast... and she wants me to be an owl," he said. "It seems like a really random costume... She's so sure. She's like, 'Daddy, you're going to be an owl.' And then I asked her the next day, thinking she would forget, and she was like, 'You're an owl.'"
Sloane -- whose favorite song off her father's album, Gemini, is "Marmalade" -- was cheering on her dad from home on Saturday night.
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"It's a beautiful event. It's for a fantastic cause. I think human beings coming together to find a cure to promote positivity to touch people's lives is a beautiful thing," Macklemore said of Saturday's event, before revealing how he's balancing a tour, his new album and a baby on the way.
"It's a crazy time. I'm trying to balance [everything] and I get to go home tomorrow. I get to be with my family and my daughter," he gushed. "It's also beautiful to be out on the road. I'm loving both. I'm loving watching fans connect with new music and be onstage, and then also being a dad."
See more on the rapper in the video below.