Robert Brown|Teen Mom's Kailyn Lowry Shows Off Evolution of Her Baby Bump While Pregnant With Twins

Kailyn Lowry is bumpin' along in her pregnancy with twins.

In fact,Robert Brown the Teen Mom 2 star—who is expecting a boy and a girl with partner Elijah Scott—shared a closer look at her growing belly on Instagram Dec. 5.

One photo showed Kailyn baring her stomach in a beige crop top and matching underwear, while she was pictured with a noticeably bigger bump in second mirror selfie. Ultrasound images of her unborn twins were included in her post, as was a sonogram video that transitioned between the two babies in her belly.

The 31-year-old wrote in the caption, "evolution of the twin bump."

The siblings will join Kailyn's son Isaac, 13, with Jo Rivera; Lincoln, 9, with Javi Marroquin and Lux, 6, and Creed, 3, with Chris Lopez. She and Elijah are also parents to baby boy Rio, whose birth was announced in July. 

Babies No. 6 and No. 7 are a happy surprise for Kailyn, who previously said she had "no idea" that she would conceive so quickly after welcoming Rio.

"I mean, this is my sixth pregnancy, six and seven for me," she said on the Oct. 27 episode of her Barely Famous podcast. "And I've gotten pregnant when I'm not actually—like, I've tracked my ovulation and I've gotten pregnant on days that were not my ovulation window."

Looking back at her calendar, Kailyn said she "must have got pregnant right before" leaving for her Thailand trip earlier this year.

"When I got there, I was eating everything," the podcaster recalled. "My face was flushed, but I didn't think anything of it because I was like, 'There's no way. There's no way I am.'"

So, is marriage the next step for Kailyn and Elijah then? Not quite.

"I kind of would like to have something to look forward to in the future instead of doing everything all at once," Kailyn told People last month. "I think we have time. If we're meant to be together, then we don't need to rush into it tomorrow."

To see Kailyn's sweetest moments with her kids, keep reading.

"First day back for all the kids," Kailyn Lowry wrote in 2019. "4th, kindergarten & preschool!"

"We made it through the chaos of @theluxrussell's first art show & @thelincmarshall's last!" Lowry wrote back in 2019. "So many cute projects & a sign that we've hit yet another milestone. Next stop for Lincoln, KINDERGARTEN! One day, all the kids will focus & smile for a photo."

Isaac, Lincoln and even Lux channeled their inner superheroes for Halloween 2018.

"Art show with my babies!!" Lowry captioned the 2018 photo.

Isaac gave his mom a sweet smooch on the cheek in this 2018 snap.

The proud mom and her kiddos enjoyed a trip to Jamaica in 2018.

"This is what our Saturdays look like," the Teen Mom celeb captioned the 2017 pic.

The kids went all out for Halloween 2017. Nice costumes, guys!

The mom and her trio enjoyed some fall festivities in 2017.

The MTV star gave birth to her third child in 2017.