“When we talk about sexual violence, what we’re really talking about is power imbalance between women and men,” she said. “We’re learning more and more how insidious that inequality is in our society.”

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Hawks and her fellow researchers, affiliated with the Cambridge Health Alliance, Harvard, and Hunter College, based their study on data from more than 13,300 women between the ages of 18 and 44. The data, which included personal interviews, came from the 2011-2017 National Survey of Family Growth, a nationally representative survey conducted by the federal Centres for Disease Control and Prevention.

The new analysis showed that 6.5 per cent of the survey respondents – or the equivalent of more than 3.3 million women – said their first sexual intercourse experience was forced, according to the researchers.

“A large number of these women were raped,” said Hawks.

The types of coercion the survey participants said they were subjected to overlapped in many cases.

Overall, 46 per cent of the women said they were physically held down, nearly 27 per cent said they were threatened physically, and more than 25 per cent said they were physically harmed in ways apart from the rape. Half of the women said the men involved were older or larger than they were. More than 56 per cent said they were verbally pressured, and 22 per cent said they had been given a drug.

The authors found numerous differences between the women who said their first sexual experience was voluntary and those who were forced or coerced.

The average age when the first experience was voluntary was 17.4 years old and the median age of the male partner was 21, much closer in age then the forced liaisons.

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Women who reported forced sexual initiation were also more likely to have negative outcomes later in life.

The women who said they were forced to have sexual intercourse were more likely to have experienced an unwanted pregnancy than women whose first experience was voluntary. The women who were forced were also more likely to have had an abortion at some time in their lives, to have never used birth control, to have engaged in illicit drug use, and to have difficulty completing tasks outside the home. They were also more apt to describe their overall health as only fair or even poor.

The researchers also found the women whose first experience was forced were likely to have gynaecological health issues later in life, including pelvic inflammatory disease, endometriosis, and problems with menstruation or ovulation.

Carol E. Tracy, executive director of the Philadelphia-based Women’s Law Project, said the study sheds needed light on the long-term health impacts of sexual assault.

“Rape is insufficiently prosecuted as a crime and practically ignored as a threat to public health. We already know sexual violence compromises health beyond injuries potentially sustained through the actual assault, but this study is important because it directly correlates sexual violence with specific long-term health issues,” Tracy said. “It’s beyond time to address rape as both a serious crime and public health issue.”

Although this new study does not include data on the types of psychological impact on the women who endured forced first sexual intercourse, sexual violence and victimisation often result in serious harm to mental health. The effects can vary even more when the victim is a child.

Edna B. Foa, director of the Centre for the Treatment and Study of Anxiety at Penn’s Perelman School of Medicine, who was not involved with the study, said 15 to 25 per cent of women who are sexually assaulted “will have post-traumatic stress disorder symptoms severe enough to interfere with their lives”.

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Foa said almost all the women she has worked with who were raped or sexually assaulted had some elements of PTSD immediately after the assault, and about 30 per cent were still experiencing PTSD symptoms a year later.

PTSD’s many symptoms can include flashbacks to the assault, nightmares, trouble sleeping or concentrating, being easily startled or frightened, overwhelming guilt or shame, difficulty maintaining close relationships, hopelessness, and avoiding places, activities, or people that bring back memories of the traumatic event.

Hawks said she hopes the findings in her study will be helpful to health care providers.

“All practitioners who are caring for women should be aware that this form of violence early in life can be an extremely traumatic event that can have lifelong consequences,” she said.

Parents, educators, and others involved with children should teach them about such issues as consent and also equality, she said.

“We’re learning more and more how insidious that inequality is in our society,” Hawks said. “We need to take a look at how we approach inequality in all its spheres.”