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Over 3.3 million women's first sexual experience was rape, according to a new study of U.S. women ages 18 to 44, which also found that these incidents were associated with health problems later in life.

The findings, published this week in the Journal of the American Medical Association, found that 6.5% of women in the U.S. — or about 1 in 16 — experienced a sexual initiation that was forced or coerced. The average woman had this experience at age 15.

These women were less likely to be white and college-educated and more likely to have incomes below the poverty level and to have been born outside the United States than women who say their first sexual encounter was voluntary, the data showed.

The study found the affected women were more likely to experience an unwanted first pregnancy, abortion, endometriosis and other reproductive health problems compared with women who reported having a voluntary experience.

Researchers used data from the 2011-2017 National Survey of Family Growth and included 13,310 women in the study. In that survey, women were asked whether their first experience of vaginal intercourse with a man was "voluntary" or "involuntary," and the authors of the study focused on the latter. Because the data was nationally representative, the authors were able to extrapolate that an estimated 3,351,733 women ages 18 to 44 did not have a voluntary first sexual experience.

And these numbers may not encompass the full scope of American women who have experienced such assaults.

"It's quite alarming, and that's just the tip of the iceberg because this study is only including women aged 18 to 44," Laura Hawks, lead author of the study, told NPR. "You can imagine that if we asked this of women of all ages, the [absolute] number would be many millions higher."

In a commentary on the study, Alison Huang of the University of California at San Francisco and Carolyn Gibson of the San Francisco Veterans Affairs Health Care System argued the data were vital for a society moving toward a more comprehensive understanding of sexual assault and more effective care for patients.

About 46% of the women who had a coerced sexual experience reported being held down during the incident — but even more reported verbal pressure (56.4%) and coercion by a partner who was larger or older (50%).

"The thing that really struck me about this paper was this idea that [for] so many of the women … it wasn't a result of physical force, the stereotypical threats," said Gibson, a psychologist. "It was the result of verbal pressure, psychological pressure, threats to end the relationship, coercion."