Hartsock revealed she's pregnant during an interview with Us Weekly. When the magazine asked her to announce her big news, Hartsock exclaimed, "We're getting a puppy!"
After letting out a little laugh, the Seattle-based fashion designer said, "I'm just kidding. We're pregnant! Yes!"
Hartsock, who is "really excited" about the baby, admitted she's been trying to hide her weight gain.
"I can't wait until I'm rounder, I guess," noted Hartsock, who is currently 13 weeks along in her pregnancy and due this fall.
The ninth-season The Bachelorette star is thankful she hasn't experienced much morning sickness so far.
"I feel really lucky. When I get sick or nauseous, I just eat! And then it's like I'm all better!" Hartsock revealed, adding that she does get odd food cravings though.
"Well, each day it's something new. So, I think that's interesting... Each day is something new. Like one day I'm like, 'Babe, I really need Italian.' And then he's like, 'Alright!' And then the next day, it's Mexican and the next it's dessert for dinner."
Hartsock, 30, said she officially found out she's pregnant during a February vacation with Siegfried in Maui.
"I kinda knew, like, 'You know something, I think I'm pregnant.' We were in Hawaii when we went for our one-year anniversary. That's when we took a test and found out on Valentine's Day!" she said.
And since that romantic trip, Siegfried, also 30, has been playing the role of doting dad-to-be.
"He's so great. He knows I have something growing inside me," Hartsock said with a laugh.
"He's just so supportive and if I'm craving ice cream, he's like, 'Okay, let's go get it!' He is such a wonderful human being, and with kids, he's really, really good. He's so nurturing. But at the same time, he has this reality of what parenting is going to be like, and that's so not the truth," Hartsock explained.
"He's like, 'Oh, our kid is never going to cry,' and, 'Oh, you see that kid over there? That's not going to be our kid.' I'm like, 'Mhmm, yup.' And then he's like, 'When our kid is in high school, he's totally going to want to hang out with me.' And I'm like, 'Okay! I don't know if that's going to happen.' But I think it's cute and endearing because I think he's in for a real treat when the baby comes."
Siegfried proposed to Hartsock during The Bachelorette's Season 9 finale, which was filmed in May 2013 and aired three months later on ABC. She selected the former professional baseball player over runner-up Drew Kenney after getting over the heartbreak of Brooks Forester unexpectedly dumping her.
Hartsock has a book coming out on May 17 called My Fairytale Wedding, which will share tips on how to plan a beautiful wedding on a budget.