"It's about awareness. Once the public knows, then they can make their own choices," she tells PEOPLE

Marisa Miller is baring it all for a cause close to her heart.

The mom-to-be — who previously posed nude while pregnant with son Gavin Lee, now 2 — is stripping down to show her support for animals in captivity as part of PETA‘s new campaign, SeaWorldofHurt.

“I’ve always been an animal lover and I do a lot with ocean conservation,” the model, 36, tells PEOPLE exclusively.

“I always see dolphins when I surf, but I didn’t have the knowledge about how they are acquired. When Blackfish came out, I saw it and I was so upset.”

For the ad, Miller shows off her baby belly while lying naked in a bathtub, mimicking the world of captured killer whales, who are confined to living in small enclosures.

“Even the tanks they do their ridiculous tricks in are way bigger than where they actually sleep,” she explains. “They’re completely confined. That’s why they have all these dental problems, because they are chewing on metal and concrete — they can’t even move.”

Marisa Miller pregnant nude PETA ad

Courtesy PETA

Although Miller understands the fascination with wildlife, she’s hoping the photo will make people think twice about supporting the parks, which, she says, keep animals in chemically treated waters.

“[The animals] are ripped apart from their family, they have to be drugged to handle the environment they’re in,” she says. “There are other ways to experience them. There is a way that you can rehabilitate the animals in an environment that they can swim and you can go check them out.”

She adds, “It’s about awareness. Once the public knows, then they can make their own choices.”

And for Miller, her choice was an obvious one: The soon-to-be mom of two admits she couldn’t turn away from the facts.

“I was so unaware of the realities about how the whales are captured, how they are separated, and their emotional capacity. This is an animal whose babies never leave their mother’s side their whole life,” she says. “So as a mother, I have a totally different perspective on relating to that.”

Despite the ad being shot during her second trimester, the timing couldn’t have been more perfect as Miller is all about embracing her baby bump. As for the nudity, she says the confidence comes with time.

“People are harsh. I think I have a little bit of a thick skin from having the career I did,” she explains.

“My mind-set has always been that it’s about being healthy [during pregnancy]. If you are being healthy and you put on 40 lbs., you put on 40 lbs.”

Having a strong support system — for Miller it’s husband Griffin Guess — helps, too. “I have a husband that is so wonderful and accepting [about] the things that aren’t so pretty that happen when you’re pregnant. It’s so not a big deal to him,” she says.

“Your body is like, ‘I know what to do!’ But I have had so much throbbing pressure, so I feel like I’m swollen, and he’s like, ‘Sweetie, you’re pregnant. It’s fine and things will go back to normal.’ It just really helps when your husband is so cool about it.”

The couple will welcome their second child later this month, and Miller — who grew up with sisters — couldn’t be more excited after revealing in January that she is expecting another baby boy.

“They are so attached to their moms I can’t even tell you. Gavin will sit on me and cuddle with me — he’s so sensitive,” she shares. “I didn’t have any brothers, so the energy is so different and he’s just so funny.”

She continues, “Some of my friends who have girls, they say a girl can sit there and color for 45 minutes, but with boys, you put on your tennis shoes and you go. The bond between mother and son is awesome. It’s the sweetest thing.”

The mom-to-be is spending her final few weeks preparing Gavin for his new role as big brother. According to the proud mama, after a period of regression — “He wanted to start nursing again,” she recalls — her firstborn “totally gets what’s going on” and is already showing off some serious skills.

“I got him a baby doll, so he gives the baby a bottle and he’ll talk to him and tell me, ‘I want to give him the toy, I want to chase him.’ He does not stop talking,” Miller says. “For a while, the baby was in his tummy, then after he got older and he really started to see my belly, then he started to understand what I was saying.”

— Anya Leon with reporting by Raha Lewis

In a statement to PEOPLE, SeaWorld responds:

As a mother who has raised two children and someone who has dedicated my life to caring for SeaWorld’s killer whales, I can tell you that there’s nothing more powerful than the connection between a mother and her calves. SeaWorld understands the importance of keeping mothers and their dependent calves together. Mothers provide support and nutrition, and we do not and would not interfere with that unless the mother or calf’s life was in danger.

Research being conducted today on our killer whales’ milk composition will help scientists understand the nutritional requirements for mothers and calves. Research into pregnancy and lactation will help scientists understand how killer whales metabolize toxins. Considering the human impact on our oceans, this research is critical for all killer whales, especially the endangered southern residents in the Pacific Northwest.

We apply these and other lessons every day. And we will keep working to do more, for killer whale moms at SeaWorld and for killer whale moms everywhere.

Kristi Burtis, Supervisor of Animal Training, SeaWorld San Diego