Melanie Notkin could not have been clearer about her desire to have children. At age 12, she was buying baby-name books. When she was 23 and interviewing for her first job, she inquired about maternity benefits — just in case.

That was 23 years ago, and while she has changed jobs in the intervening years, she still has never used maternity benefits. And she doesn’t know if she ever will.

Although the number of childless women is increasing steadily, many of the assumptions about why they don’t have children — primary among them that they don’t want kids — are being proven false. Some waited too long and face fertility problems, while others have struggled to find a mate. They are childless by chance, not by choice.

Fifteen percent of women in the 40- to 44-year-old age group were childless in 2014, up from 10 percent in the 1970s, according to a recent Pew Research Center report.

A frequently cited 2006 study in the Journal of Marriage and Family suggests that about 40 percent of the 1.6 million childless women ages 40 to 44 are childless because of fertility problems. About 16 percent still expect to have children, perhaps because they’re actively trying. But no one really knows how many of the remaining 44 percent, who are presumably fertile but expect no children, are childless by choice and how many are childless because they lack a partner. Studies haven’t been designed to answer that question.

“It’s an important question,” said Gladys Martinez, a statistician at the Centers for Disease Control and Prevention. “Being able to know if women are delaying childbearing because they haven’t found the right partner — that’s a new path that we haven’t studied before.”

Notkin, who has never married, is author of the book “Otherhood: Modern Women Finding a New Kind of Happiness” (Seal Press). In her book, she describes her experience as a successful single woman in New York City and decries stereotypes and misconceptions, among them that childless single women don’t like kids or are comically inept when it comes to dealing with them; that they’re too picky in love — or not picky enough; that they’re too careless (about their fertility) or too serious (about their careers).

“It’s the first time, certainly with any frame of reference that any of us has, where there’s a large group of women in their mid-30s and early 40s who haven’t found a partner, and I think that a lot of people make assumptions about why that person is that way,” said Lori Gottlieb, author of the bestseller “Marry Him: The Case for Settling for Mr. Good Enough.”

“They’re sort of projecting their own fears and psychology onto it.”

Friends misunderstand

Notkin’s book has gotten a range of responses, with some reviewers saying it’s too downbeat or too focused on Notkin’s cohort of glamorous Manhattan singles.

Others have welcomed the book as an honest and insightful take on a taboo topic.

“It’s like she read my mind on certain things,” said Shelli Simontacchi, a paralegal in Charlotte, N.C., who is 43 and single with no kids because she hasn’t found the right partner.

She said she went through a period of mourning the life she expected, and she sometimes feels misunderstood by friends and acquaintances. But she has been able to travel to Hawaii, Florida, London and the Caribbean. She enjoys her job, adores her niece and nephew and has a great circle of friends, she said.

“I feel complete,” she said. “I’m a whole person, regardless of whether I’m married or not.”

Notkin writes that she fell in love in her 20s with a man who broke up with her. In her mid-30s, when many of her friends were getting married and having kids, she would lie awake at night wondering where she’d gone wrong.

“Where is your baby?” she asked herself. “Where is your love? Why is this so hard?” The sight of a little boy giving his mom a construction paper butterfly brought her to tears.

Moving forward

Her advice to other women in that situation is to move forward. She went to Paris for work. She floated on the Dead Sea. She started her own business, Savvy Auntie, which caters to women who dote on the children of relatives and loved ones.

The website includes an online community to find ideas and support. Her 2011 book, “Savvy Auntie: The Ultimate Guide for Cool Aunts, Great-Aunts, Godmothers, and All Women Who Love Kids” was a bestseller, and that year she wrote a Huffington Post essay on childlessness that landed her on CNN.

Notkin said the pain of not having a child can still resurface at times, but it’s no longer overwhelming.

“It’s not the life I expected, but in many ways it’s beyond my expectations,” she said.

“I never expected to be an author; I’ve written two books. The career that I’ve built is really an extension of my life circumstances and all the wonderful and honest and vulnerable and celebratory things about that life. I enjoy dating much more — even the bad dates are great dates because you’re in the moment and you’re enjoying it and you’re out. And I love my friends. My friends are the family I choose, and we are there for each other and support each other in the highs and the lows and, more important, the everydays.

“And of course my nieces and nephew bring me extraordinary joy. So it’s all the pieces of my life that I’ve built through experience, and time, and effort and the nurturing of my relationships.”