The MTV star posted an image to Instagram overnight showing her hand resting on her stomach.
Her caption read: "18 weeks as of midnight and six more days until we know if it's a boy or a girl! Can't wait. So excited to meet our little monkey!!"
Catelynn's post was favourited more than 40,000 times with hundreds of fans sending their well wishes her way.
The young star announced her pregnancy at the four-month mark last week, following widespread rumours that she and long-time partner Tyler Baltierra were expecting.
Catelynn and Tyler rose to fame on 16 And Pregnant and then Teen Mom, which documented Catelynn's first pregnancy when she was sixteen. Viewers watched as the couple made the difficult decision to place their daughter up for adoption as they felt it was in her best interests.
"We couldn't give Carly the future she deserved and so we chose adoption. We stand behind our decision," said Catelynn recently.
"But I'm not 16 any more. We're ready for a baby."
Their second pregnancy news came shortly after they celebrated their nine-year anniversary. They met at the age of twelve in school. Marking the occasion, Tyler said: "It's been the best nine years of my life with Catelynn. Happy anniversary babe! Love you more than words can explain."
Ever since the announcement, fans have been swamping them with congratulatory messages.
One wrote on Instagram yesterday: "Congratulations to you and Tyler, you're both gonna be fantastic parents. You truly are an inspiration to a lot of people and I wish you both all the best with this pregnancy."
Catelynn's pregnancy news came shortly after another Teen Mom star, Jenelle Evans, welcomed her second child.
Jenelle and boyfriend Nathan Griffith are parents to a baby son named Kaiser. Jenelle is also mum to son Jace, who was featured on Teen Mom.
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