(CNN) More than 3.3 million American women ages 18 to 44 were raped the first time they had sexual intercourse, according to a new study, and "all demographic groups reported substantial roles of forced sexual initiation."

The study, published Monday in the medical journal JAMA Internal Medicine , notes the World Health Organization recognizes forced sexual initiation -- "an unwanted first sexual intercourse that is physically forced or coerced" -- as a distinct form of sexual violence.

"We feel it is accurate to describe these events as rape," said Dr. Laura Hawks, a primary care physician and research fellow at Harvard Medical School who co-authored the study.

The study says that 6.5% of women surveyed had an unwanted first sexual intercourse that was forced or coerced and it "appears to be common." Researchers estimated that to be 1 in 16 US women.

The average age of women who experienced forced sexual initiation was 15.6. The average age of the partner or assailant at the time was 6 years older. Among women whose first sexual intercourse was voluntary, the average age was 17.4.

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