The number of Americans who consider themselves pro-life has climbed significantly -- a shift that some doctors and abortion opponents say may be due to advances in the use of ultrasound, which allows pregnant women to see images of their babies before they're born.

"Ultrasound used to be less available, very grainy. Now the baby is very clear, very distinct," said Charmaine Yoest of Americans United for Life.

A Gallup poll conducted May 7-10 found that a majority of Americans consider themselves "pro-life" for the first time since Gallup began polling the issue in 1995.

Fifty-one percent of those surveyed said they were pro-life, while 42 percent identified themselves as pro-choice -- a startling shift from just one year ago when 50 percent said they were pro-choice and 44 percent said they were pro-life.

The change, doctors say, may be directly linked to the frequency and improved quality of ultrasound -- a medical imaging technique that allows doctors to view the size, structure and vital organs of a fetus.

Massachusetts Dr. Eric J. Keroack told FOX News that during a two-year study 75 percent of his patients who were unsure about terminating a pregnancy decided not to have an abortion after they opted to view the ultrasound images.

Abortion opponents in legislatures across the nation are now lobbying for laws to require that women seeking an abortion view an ultrasound -- or at least have the option to do so -- in an effort to decrease the abortion rate.

Six states currently require verbal counseling or written materials to include information on accessing ultrasound services, and 12 states regulate the provision of ultrasound by abortion providers.

Legislation passed in Oklahoma last year requires abortion providers to perform an ultrasound on every woman seeking to terminate her pregnancy and to move the monitor into a position that makes it easy for women to see. The bill is being challenged by the Center for Reproductive Rights, which represents abortion clinics. 

Doctors must provide a "simultaneous description of what the ultrasound is depicting," which "shall include the dimensions of the embryo or fetus, the presence of cardiac activity, if present and viewable, and presence of external members and internal organs, if present and viewable," the legislation stipulates. 

Women may avert their eyes if they choose to, the bill states, and "neither the physician nor the pregnant woman shall be subject to any penalty if she refuses to look at the presented ultrasound images."

In Nebraska, lawmakers advanced a measure last week requiring abortion providers to display ultrasound images of fetuses in a way that patients can easily view them. Lawmakers voted 37-5 Thursday to push the measure to the second round of debate.

"It gives young women who are struggling with that decision the option to view it and hopefully see a beating heart and the development of the child," Nebraska state Sen. John Harms told FOXNews.com.

"I've had correspondence with women who've gone through an abortion and who've said that if they had the option of seeing the ultrasound, they might not have made that decision," he said.

But abortion rights advocates are strongly opposed to such legislation, arguing that technology is being used to generate guilt among women who choose to abort.

"The other side has made it pretty clear that this is a strategy to get women to change their minds," Gretchen Borchelt, senior counsel at the National Women's Law Center, told FOXNews.com.

While Borchelt said she's not opposed to giving women the option of viewing an ultrasound, she's vehemently against requiring women to view the images.

"We do not support that type of legislation. That's interference," she said.

Nancy Keenan, president of NARAL Pro-Choice America, also expressed opposition to a law that forces pregnant women to view an ultrasound. "Women would rather talk to their doctor about information they need to make private, personal medical decisions -- this is not a place for interference by politicians," she said.

"We do not oppose legislation that allows a woman the option to view an ultrasound that she has requested or that her doctor determines is medically necessary," Keenan said. 

FOX News' Shannon Bream and The Associated Press contributed to this report.