Until recently, the longest period Hilary Swank had spent apart from her 12-month-old twins was four hours. At the end of January, she flew to New York for a night to celebrate the 60th birthday of the actress Mariska Hargitay.

“That was a big moment, leaving them for that bit of time,” Swank, 49, said, “But I think it’s also healthy for us.”

The actor spoke with me while sitting in the New York headquarters of HealthyBaby, a startup that sells plant-based, biodegradable diapers and other products; late last month, it opened the Nest, a play space for babies and new parents.

Swank, who has a partial stake in the company, serves as the brand’s chief innovation officer. She also serves as a spokeswoman and helps the team come up with products.

Like others who are new to motherhood, on the day we met, Swank was brimming with ideas about folate and the “fourth trimester.” She did not previously see herself becoming an advocate for infant and maternal health, she said, but she always wanted to have children.

“But, you know, then you start focusing on your career and you have other dreams and desires that you want to see come to fruition,” she said. She won best actress Academy Awards for 1999’s Boys Don’t Cry and 2004’s Million Dollar Baby.

“I wanted, when I did have kids, to be able to give my focus to them and not have it pulled away. I mean, it would have been great to have had kids a little earlier but then, I didn’t have the right partner. There were a lot of factors,” she said.

“I could have gotten a donor and had one by myself,” she added. “I probably would have done that had I not found the right partner, but I did, in the nick of time.”

Swank was previously married to the actor Chad Lowe; they divorced in 2007. In 2018, she married Philip Schneider, an entrepreneur and philanthropist. While she didn’t go into any details about how challenging (or not) the process of getting pregnant was, she did say that she didn’t consider using a surrogate.

“I know some people can’t wait for it to be over, but I loved being pregnant,” she said, especially because she was carrying twins. “I always kind of felt I was going to have twins my whole life. My grandmother's a twin. My husband's grandmother was a twin and I just always kind of felt like … Like, I don't know if I manifested that. I was very happy to hear I was having twins. It was a very joyous moment.”

Swank has spent much of the past year at her Bellingham, Wash., home with Aya, who is named after a Syrian refugee that she and Mr. Schneider met in Lebanon in 2016, and Ohm, for the Sanskrit mantra, “the sound that brings people together,” she said. She forged friendships in a moms group she found through the actress Emmy Rossum. ”Even the moms group was like, ‘What do I do? I just had a baby, can anyone tell me what you do?’”

The actor spent her pregnancy filming Alaska Daily, an ABC series that has since been canceled. “I worked until the second week of my third trimester,” she said, “on my feet, long hours, memorizing lines.” She also worked on a film that came out in February, Ordinary Angels.

While pregnant, she found herself Googling many of the things that keep expectant parents up at night, from pre and postnatal traditions to brain health in babies and toxins in diapers. And like so many new parents, she continues to Google for better or worse.

“At three o’clock in the morning, I’ll get a text from her about microplastics,” said Shazi Visram, the founder and C.E.O. of HealthyBaby.

“They show up in your uterus,” said Ms. Swank, referring to a 2020 study that found microplastic particles in placentas.

“She’s a research freak, like me,” Ms. Visram went on. In addition to HealthyBaby, Visram, 47, is the founder of Happy Family Organics, which sells baby food. The two women met last year, when Ms. Swank was pregnant and trying to figure out what brand of diapers to buy. Ms. Swank posted about the brand on Instagram and messaged Ms. Visram; their relationship developed from there.

Visram and Swank work in tandem on everything from new product ideation to what goes on the HealthyBaby website. As Visram showed Swank a guide that would go out to customers who sign up for a diaper subscription, she joked, “It doesn’t go live until Hilary approves it. We’re not playing here.”

Newly aware of how full of doubts new mothers can be, Swank has made it her mission to let women like her know that it’s okay to take a beat, to give yourself grace, “to start over again tomorrow, or even in the next moment,” she said.

“Which every new mom needs to hear,” she added. “You get all caught up, like, ‘Am I doing it wrong?’ But we really have to trust our instincts.”