"So this is going to be my final season, my final reunion," Chelsea, 29, told her "Teen Mom 2" co-stars Kailyn Lowry, Leah Messer, Briana DeJesus and Jade Cline. "And it is definitely bittersweet. And it's weird that the last one is here like this, and we're not all together."
When asked by reunion co-host Dr. Drew about her decision to leave the series, Chelsea said, "I feel like, just watching this last season, I was getting a bad feeling in my stomach. It just feels like it's just time to close the chapter."
"And I'm almost 30, I'm having my fourth baby -- it just feels like the right time," added Chelsea, who is expecting her third child with husband Cole DeBoer and also shares daughter Aubree, 11, with ex-boyfriend Adam Lind.
As Dr. Drew voiced that he's "both pissed and sad" over Chelsea's departure, Chelsea's co-stars were encouraged to share their thoughts as well.
"More power to you. Good luck on your journey with you and your family. I wish you nothing but the best," said Briana, 26.
Both Leah, 28, and Jade, 23, said they weren't expecting Chelsea's decision, while Lowry, 28, wasn't "entirely shocked."
"It's sad," explained Leah. "It's definitely bittersweet, but I think that Chelsea and her family get to make the best decision for their family at the end of the day and I wish them nothing but the best."
Jade added, "I'm really surprised. Honestly, I didn't like, expect it, especially since she's having a new baby and stuff. It seemed like there was a lot of stuff going on, but I totally understand wanting to step back from the reality TV world."
On the other hand, Kail -- who, like Leah, has starred alongside Chelsea since Season 1 -- admitted she wasn't too surprised by the news.
"I'm not entirely shocked, just because I do talk to Chelsea pretty often, so we've had conversations here and there," Kail told the group. "But, you know, I definitely told Chelsea, 'This is obviously the right decision for you if you feel like there's a weight lifted off your shoulders.'"
"I do think it's going to be a little bit weird of a dynamic without her, because we've been doing this together for 10 years," Kail added. "I'm really excited to see what opportunities she has when she moves on from this."
Back in November, Chelsea confirmed reports that she's exiting "Teen Mom 2" after 10 seasons.
"MTV's Teen Mom 2 has been a big part of my life for almost 11 years. After much thought and discussion with my family and friends, Cole and I have decided that this season will be our last," she shared on Instagram.
"We are forever thankful to MTV and our crew, who are like family to us. We're parting on the best of terms and will stay in touch long after this," she continued. "We're proud to have been able to share our story and are so grateful to the fans who have followed our journey from the beginning."
She said the family's next chapter will "focus on developing our brand and taking things to the next level with new endeavors and expanding family businesses."
Chelsea said the family is "so excited for this next phase" and added she hopes their fans will still "be a part of it in some way."
Viewers were first introduced to Chelsea on MTV's "16 And Pregnant" as she navigated life with newborn daughter Aubree and ex-boyfriend Adam Lind. She went on to star in "Teen Mom 2" in 2011 alongside Jenelle Evans, Kailyn Lowry and Leah Messer.
Briana DeJesus and Jade Cline were added later as series regulars.
Fans followed Chelsea as she met and married Cole DeBoer during her run on the reality series. The pair share daughter Layne, 2, and son Watson, 3. In August, they announced they are expecting their third child together. Cole also helps raise Aubree, now 11.
Part 1 of the "Teen Mom 2" Season 10 reunion airs Tuesday at 8 p.m. ET on MTV.