Catherine Lowe Reveals Second Pregnancy With Husband Sean Was a 'Surprise' (Exclusive)

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"He just looked sexy!"

That's the adorable reason Catherine Lowe gave as to why she and husband Sean are expecting again! 

ET's Lauren Zima exclusively spoke with the Bachelor alum at Fit180 gym in Dallas, Texas, where she opened up about her body transformation after her first pregnancy (with son Samuel)  -- and getting pregnant with baby No. 2.

"I mean, I looked really good right before Samuel was born, and we were both looking good, so, you know, what happens, happens," she confessed, revealing that Samuel was unplanned. "There was no planning involved [for our second child], so [that's] probably exactly what had happened." 

"It's probably good that it was a surprise, because I probably would have been like, 'I don't know, my body looks so good,'" added Catherine, who has totally transformed her physique with the help of her trainer, Julie Hoang, after welcoming son Samuel in July 2016. Our exclusive interview with Lowe and Hoang airs Monday night on ET, where they'll explain how Catherine lost more than 50 pounds by exercising just twice a week. 

EXCLUSIVE: Sean and Catherine Lowe Open Up About Parenting & Wanting More Kids: We 'Have a Heart for Adoption'

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BABA NUMBA TWO 👶🏼

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As for the news of baby No. 2, Lowe says: "I knew I wanted a lot of kids, I just didn't know exactly when I'd be ready, because it's a lot. Guys can be ready easily, because you don't have to do anything for a long time! But immediately when it happens, everything changes [for women], so I had to be mentally prepared."

Catherine also shared that she and Sean, who on Friday celebrated the 5-year anniversary of their engagement on season 17 of The Bachelor, now have a tradition when it comes to confirming their pregnancies.

"The first time around, I peed on the stick and I went to bed. He checked the stick, and I said, 'It's negative, right?' And he's like, 'Yeah,' and then he said, 'Well, when I say negative, it means positive,' so we had this moment of 'Oh my gosh, this is nuts!'" she explained. "Then this time, he checked again, [then] he told me, and so I think that's just the tradition we have. He's gonna check! ... I want him to feel like he's having [a baby] as much as I am."

EXCLUSIVE: Sean and Catherine Lowe on Why They're the Only 'Bachelor' Couple to Get (and Stay) Married

"My husband is so sweet and I can't imagine having just a nicer husband," she continued. "I could look like doo doo and he would say I'm cute!"

Catherine explains that, for her, hitting the gym is "really about my confidence ... He can tell that I feel really good about myself, and that's sexy to him... For a really long time, [I felt] 'Oh, my boobs are too big and I'm too big and I can't fit into stuff and I'm tired' ... but after that changed, he was like, 'Oh, there's my wife again,' so it was very attractive to him that I was just even in a better place." 

"Obviously [working out] is important to me... we really are a great team when it comes to parenting. He's always willing to say, 'You know what? I'll take the boy and I'll go do whatever,' because he knows that when I work out and I get my body back, then I'll feel better," Catherine said. 

The Lowe Co. founder also leans on Hoang for support both in and out of the gym. 

"I trust her so much. She knows what it's like to be a mom. She's really curated a great plan for me that covers all bases, that makes me feel really comfortable, and we got here," Catherine raved of Hoang, whom she's been training with for a year and a half. "She's probably the person that I talk to more than anybody else besides Sean."

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"When you work out, your guard comes down, because you're tired. So things come out and we get along so well... but I'm sworn to secrecy," Hoang agreed. "She's great about just trusting the process when she comes in." 

The Lowes don't plan to find out whether they're having a boy or a girl (they didn't with Samuel, either), but Catherine tells ET she's due in mid-to-late May. 

To find out how Catherine and Hoang totally changed Catherine's body after baby and how they're adjusting her workouts now that she's pregnant again, tune in to Entertainment Tonight on Monday night! 

Sean and Catherine opened up about expanding their family while speaking with ET earlier this year. Watch below.