Room for one more! Teen Mom OG's Maci Bookout announced on Valentine's Day that she is expecting her second child with fiancé Taylor McKinney, and the reality star tells Us Weekly exclusively that the people close to her were completely stunned when they learned the news.

Bookout — who shares daughter Jayde, 8 months, with McKinney, and son Bentley, 7, with ex Ryan Edwards — tells Us that she found out about the pregnancy in January, just two days after McKinney proposed.

"I was at the doctor, ironically, to get my birth control prescription refilled!" Bookout, 24, says.

Her doctor read her test results and informed the MTV star that she couldn't get birth control because she was already pregnant, which Bookout initially refused to believe.

"I laughed at her!" Bookout says. "I mean, she's the doctor that delivered Jayde, so I was like, 'Ha ha, that's a funny joke — but it's not really that funny!' And she was like, 'No, seriously!' And I was like, 'Oh, OK — well, give me 10 minutes to not freak out … You're wrong. You've got the wrong results, you mixed them up.' But no, she was serious!"

Bookout, who is expecting a boy on May 30, reveals that her family and friends have similarly been taken aback by the big news.

"Everyone is shocked," she says. "I mean, even we were completely speechless, and every time I shared the news with someone, I said, 'OK, are you sitting down?' And then I said, 'Feel free to say 'holy s–t' or congratulations because both are fine.' Because that's pretty much how I've been the whole time!"

Bookout says the bundle of joy won't dramatically alter the couple's wedding plans, as they are planning for an October ceremony. And while the social-media consultant says she and McKinney, 26, are both very excited about the baby, the pair "absolutely" don't plan on welcoming any more children after this one.

"There are going to be some permanent birth control decisions made immediately after birth," she says.

Teen Mom OG's two-hour season finale airs on MTV on Monday, February 22, at 10 p.m. EST.