For decades, researchers and physicians tended to think about pregnancies as either planned or unplanned. But new data reveals that for a significant group of women, their feelings don’t neatly fit into one category or another. As many as one-fifth of women who become pregnant aren’t sure whether they want a baby.

This fact may reshape how doctors and policymakers think about family planning. For women who are unsure, it doesn’t seem enough for physicians to counsel them on pregnancy prevention or prenatal care.

“In the past we thought of it as binary, you want to be pregnant or not, so you need contraception or a prenatal vitamin,” said Maria Isabel Rodriguez, an obstetrician-gynecologist at Oregon Health and Science University. “But it’s more of a continuum.”

The new data comes from a recent change in the Centers for Disease Control and Prevention’s big survey of new mothers, now allowing them to answer a question about their pregnancy desires by saying “I wasn’t sure.” It shows that some women want to avoid making a decision about becoming pregnant, or have strong but mixed feelings about it. A new analysis of the 2014 results from the Guttmacher Institute combined these results with data from abortion providers. It found that in 9 percent to 19 percent of pregnancies, the woman “wasn’t sure” what she wanted at the time.

Other research has asked people whether they want to have children, or have more children. An analysis of 33 studies of fertility intentions in developed countries found that roughly one-fifth to one-third of women said they weren’t sure.

Women ambivalent about pregnancy tend to be younger, wrestling with financial strains and life plans, or older, nearing the end of their fertile years. They are more likely to be black than white. They are more likely to already have at least two children, said Isaac Maddow-Zimmet, a senior research associate at Guttmacher, describing an analysis that has not yet been published.

The research confirms that many unplanned pregnancies can nevertheless become wanted as women’s feelings about pregnancy evolve.

Kristin Jennings had been told she might be infertile, and had decided that children weren’t for her. But a year ago, when she and her husband felt financially secure, they decided to see what would happen if she stopped taking contraception. To her shock, a pregnancy test came up positive within weeks.

“It was one of those weird moments where I know if we were actually trying and it was something we really wanted and were really hoping for, I would have been really excited,” she said. “But when I was looking down, I was like, ‘Oh, this is unexpected.’ ”

A few weeks later, Jennings, 33, was offered a big promotion at her job at an industrial company in suburban Cleveland. “Had I not gotten pregnant that month specifically, I probably would have told my husband: ‘I can’t do this. I need to focus on my job.’ ”

Jennings’ mixed feelings have turned to enthusiasm over time. She loves her 11-month-old daughter, Jade, and is thriving in her new job. But she said that she felt lonely in her ambivalence at the time. “I’m surrounded by women who just ache to be mothers,” she said. “I don’t know anybody with my situation.”

The best care for an ambivalent woman, besides guidance about prenatal health, might be a form of birth control that is more easily reversible. When women say they’re unsure, Rodriguez talks to them about things like taking folic acid for fetal health, just in case, and about options for emergency contraception or abortion. She also asks questions that might uncover stressors related to family planning, like relationship problems or uncertainty about age and fertility.

The new evidence, she said, shows the importance of providing continuous medical care to women of reproductive age, not just when they’re pregnant.