Parenthood was once thought to be inevitable — a destiny — for healthy fertile adults.

No more. Many people are opting out, a life choice that still provokes debate.

The number of babies born in the U.S. last year fell to the lowest level in 32 years, with younger women especially having fewer kids. Americans are now having fewer children than it takes to replace the population, a trend mirrored in other countries.

It doesn’t surprise Amy Blackstone, a sociology professor at the University of Maine and author of the new book, “Childfree by Choice: The Movement Redefining Family and Creating a New Age of Independence.”

She and her husband Lance decided not to have children years ago after simply not feeling the pull towards parenthood. They check in with each other every year to make sure they’re still both on board about opting out. Famous women who've also made the choice include Ina Garten, Cameron Diaz and Helen Mirren.

Amy and Lance Blackstone got married in 1995. For years when people asked her when she was going to have kids, she responded, "I'm not ready yet." But then she realized she just didn't want children. Courtesy Amy Blackstone

For her book, Blackstone interviewed 70 child-free men and women and surveyed more than 700 about their experiences. She found women still feel the brunt of the stigma.

“Men sort of get a pat on the back and there’s more joking about, ‘Aren’t you lucky that you dodged this bullet?’” Blackstone, 47, told TODAY.

“Whereas for women, it’s the, ‘Oh, you poor thing, I can’t believe you’re missing out on the most meaningful experience that a woman can have. You must be so sad and lonely.’”

Here are eight more of her findings:

1. Many of the top reasons for skipping parenthood are especially true for millennials

Blackstone: The cost of living and having a baby — we know that’s a particularly difficult issue for millennials who are facing all kinds of college debt. Certainly, having a child has an impact on the environment and I know that’s a reason that millennials have shared for their choice.

Other top reasons include the desire for autonomy, spontaneity, freedom and the ability to travel.

2. Many child-free people want to focus on the relationships they already have

Blackstone: This quote from a 44-year-old married woman resonated for me: “I worry that if I had a child I’d become a terrible partner because I’d be so focused on being a good parent.”

This doesn’t mean child-free people necessarily have perfect relationships or better relationships than parents. But in my own case, I do recognize that I would be giving something up in terms of my ability to feel close with my partner and nurture that relationship.

3. It’s not a ‘selfish’ choice

Blackstone: In 2015, Pope Francis said, “The choice to not have children is selfish.”

If we’re going to put that label on the child-free, then it’s a label that needs to be shared across any group of people who’ve made a choice about the life that they know is right for them.

But isn’t that what we’re all doing? Even parents, if you ask them why they had children, would tell you that they wanted kids because that’s the life that they envisioned for themselves.

Alternatively, we can choose to abandon the “selfish” label and decide it’s OK for people to make a life choice that is best for them, whether that be parenthood or non-parenthood. Maybe neither choice is selfish.

There’s an impression that child-free people don’t give back, or aren’t giving to their communities or making a difference in the world. Frankly, nothing could be further from the truth. We know from research that the child-free are involved in their communities — they’re about as likely as parents to volunteer.

4. Many child-free people do like kids

Blackstone: A quarter of the child-free people I interviewed actually chose careers that require them to be involved and make a difference in children’s lives. Many of them are teachers, social workers, pediatricians. There are all kinds of ways the child-free are engaged in kids’ lives and made a choice to do that.

Some child-free people don’t like children and in that case, the last thing we want to do is push them into becoming parents.

5. Child-free people don’t have regret down the road

Blackstone: I have not talked with anyone who feels regret about their choice.

I have had family members who I know have been worried for me, but we should accept when people tell us they don’t want to have children. Parenthood is a role that is best fulfilled when it’s one that’s chosen. It takes a lot to be a good parent so if somebody doesn’t feel that pull, that’s perfectly OK.

6. Child-free people are fulfilled and happy

Blackstone: When people say we are missing out on something, that’s absolutely true. But I would also turn it around and say it’s possible that parents are missing out on some aspects of the lives that child-free people enjoy. We can’t do it all — it’s impossible to have every life experience.

So yes, we will miss some experiences, but I don’t think that because that is true, that it necessarily follows we’re unhappy. I’m very happy with my decision. My husband and I have a life that we love.

The Blackstones scuba dive in Roatan. "I doubt I would have been capable of balancing motherhood with the sort of connection with my partner that I desire," Amy Blackstone writes in her new book. Courtesy Amy Blackstone

7. ‘Who will care for you in old age?’ and ‘Won’t you be lonely?’ are questions for everyone

Blackstone: These are questions that we all should be thinking about as we age, whether we have children or not.

In terms of the child-free, many have been creating a nest egg to help them be able to provide for themselves in their old age. And we’re seeing more and more examples of “The Golden Girls”-style living where older adults are sharing households with each other.

It’s a mistake to assume having children means one will have a person to care for them in their old age. Not every adult child cares for their aging parents, research shows.

8. A child-free household is a family

Blackstone: I would love it if we came to understand that the child-free have families. I count my husband and me as a family.

Child-free families fulfill the same functions that families with children do. We create households as a safe space that provides an emotional connection and an opportunity to recharge. We engage in “social reproduction,” which involves anything that people do to help rear the next generation. For the child-free, that means being mentors and friends to children.