Pregnant Heidi Montag Pratt Wears Red Bikini as She Gets in 'Pregnancy Workout' on the Beach

Heidi Montag/Instagram

Heidi Montag Pratt is staying active throughout her pregnancy.

On Thursday, The Hills: New Beginnings star, 35, shared a video from her family beach vacation. In the clip, Heidi tells husband Spencer Pratt, who films, that she's "going to do a little pregnancy beach workout here."

Heidi begins doing squats, while their son Gunner Stone, 4, starts jumping around before his dad encourages him to start doing push-ups. He adorably obliges with mixed results as the waves lightly wash over him.

"Pregnancy workout," she captioned the cute video.

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.

While talking to PEOPLE about her home organization as she awaits her second baby, The Hills: New Beginnings star, 35, shared how meaningful the months ahead will be as she and husband Spencer Pratt shift to being a family of four.

"I'm so excited for Gunner to have a sibling," Heidi told PEOPLE. "He loves babies, he's been asking me for a sibling for so long. So to have this new love and this new relationship that he gets to have and that we all get to have is amazing."

Heidi is also grateful for the opportunity to experience motherhood a second time around. "I'm excited to be able to go back and have a baby again," she said, adding, "to be able to enjoy all those moments."

RELATED: Heidi Montag Shows Off Baby Bump in Black Maxi Dress: '5 Months!'

heidimontag/Instagram

Heidi and husband Spencer Pratt confirmed to PEOPLE in June that their second baby is on the way. Though she isn't due until December, she's expecting her little one will make an early arrival.

"Gunner was three weeks early, so I'm anticipating that to be the norm. I've asked my doctor like, 'is that a reoccurring situation?' Basically, she said yes," Heidi explains. "So if you're body naturally kicks out babies early, you'll tend to do that."

Heidi says she is "beyond excited" to welcome her new baby during the holiday season. "Just to have that balance of having two also," she adds. "Being a mom of one has been way more challenging than I anticipated, but so much more rewarding as well."