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Nearly 99 percent of women went ahead with an abortion after voluntarily viewing an ultrasound image of the fetus beforehand, according to a large new U.S. study.

Based on medical records for more than 15,000 women seeking abortion at Los Angeles Planned Parenthood clinics, researchers found that only a small fraction of the women changed their minds after seeing the image.

"This study was motivated in large part by the current political and popular interest in what role ultrasound viewing plays in women's decisions about abortion," said one of the authors, Katrina Kimport at the University of California San Francisco School of Medicine.

Ten states have enacted laws that require doctors to perform ultrasounds before abortions, and three of those require the woman to view the image during the ultrasound. The others require doctors to offer women the option of viewing it.

A 2011 North Carolina ultrasound law, considered one of the strictest in the nation, was struck down by a federal judge earlier this month because it forced doctors to explain the image while showing it to the patient. The U.S. District Court held that forced speech to be unconstitutional.

Advocates for ultrasound laws base the requirement on the idea that showing a woman the image of her fetus might cause her to have a change of heart about terminating the pregnancy.

Kimport said there's been a lot of discussion about what effects viewing would have on women who are seeking or considering abortions, but there was very little research on what actually happens.

"We were interested in bringing in an empirical perspective to these conversations," she told Reuters Health.

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The researchers reviewed medical records from 15,575 visits at 19 Planned Parenthood clinics in Los Angeles during 2011. These facilities routinely perform ultrasounds before abortion procedures and regularly ask the patients if they want to see the images. It's also standard practice to ask each patient how confident she is about her decision to terminate the pregnancy.

Responses to both of these questions are noted in the patient's electronic medical record, according to the researchers.

Kimport and her colleagues analyzed those records and found that most women (85.4 percent) said they were certain they had made the right decision to have the abortion. A smaller number (7.4 percent) were classified as having medium or low levels of certainty about getting the procedure.

Although all of the women included in the study had ultrasounds, less than half (42.5 percent) chose to see the image.

A total of 98.8 percent of the planned abortions took place. Among women who did not view their ultrasounds, 99 percent went through with the procedure. Among those who did view the images, 98.4 percent of the women had abortions.

The images appeared to have the greatest effect among women who had expressed low or medium certainty about the procedure. In that group, those who viewed the ultrasounds were slightly less likely to go through with the abortion: 95.2 percent did have the procedure, compared to 97.5 percent of uncertain women who did not view the ultrasound.

Women who expressed high certainty about their decision showed no differences, whether or not they had viewed the ultrasound.

The study team also looked at other factors that might affect the women's decisions and found that how far along the pregnancy was turned out to be much more significant than whether a woman viewed ultrasound images.

Women at 17 to 19 weeks of gestation were almost 20 times as likely to back out of the abortion compared to women at less than 9 weeks gestation.

Based on their results, the authors conclude in the journal Obstetrics and Gynecology that mandatory viewing of ultrasound images is not likely to significantly influence how many women get abortions.

Women should be offered the opportunity to view their ultrasound before an abortion, the authors write, but mandatory viewing should be avoided.

"I think that when we start moving from a place where it's about a patient decision to a legal requirement, we're moving into the space that the literature around health suggests is going to have negative effects on women's health outcomes," Kimport said.

The researchers did not study other possible effects of viewing, or not viewing, ultrasound images, such as their emotional impact on the women.

Another unknown is what might happen when women feel forced to view ultrasounds rather than making the choice to see the images, the study authors write.

"There's extensive literature looking at a range of health-related issues that show that when a patient feels they have made a decision - and they feel engaged in the decision-making - they have better satisfaction with their care and better health outcomes," Kimport said.