(CNN) For most women, access to reproductive and sexual health services is crucial to well-being. A new study highlights the difficulties surrounding access to one aspect of health care -- abortion services -- faced by women in some areas of the United States.

Overall, between 2011 and 2014, the average distance a woman traveled to terminate a pregnancy increased by just 0.2 miles, yet spatial analysis revealed vast differences in miles traveled depending on the state and region where women lived, according to an analysis published Tuesday in The Lancet Public Health journal

On average, women in the United States traveled about 11 miles to obtain an abortion in 2014. The distance to obtain an abortion actually decreased in nine states, Guttmacher Institute researchers found. However, seven states saw an increase during that period, and women in rural areas typically traveled the farthest. In remote regions of South Dakota, for example, women traveled in excess of 330 miles to terminate a pregnancy.

Overall, the analysis revealed that women across the nation do not have equal access to abortion.

"Disparities have persisted since at least 2000," said Jonathan Bearak, lead author and senior research scientist at Guttmacher, a research and policy organization. In particular, he said, the disparities persist "between women in urban areas versus rural."

Distance is 'key measure of access'

In 2011, the most recent data available, about 45% (or 2.8 million) of the 6.1 million pregnancies in the United States were unintended, and 42% of these unintended pregnancies ended in abortion, according to the Guttmacher Institute.

For the study, Bearak and his co-authors analyzed geographic access to abortion.

"How far a woman has to travel for an abortion is a key measure of access," Bearak said. Other measures include restrictive laws and financial constraints.

To analyze how far women travel to terminate a pregnancy across the nation, the researchers began with data on the location of abortion providers and women. The information on women was based on census block groups, Bearak said: "That is the smallest publicly available geographic unit." Within states are counties, within counties are census tracts, and within tracts are block groups.

The research team matched each census block group to the geographic coordinates for the nearest abortion provider and calculated the driving distance from the block group to the provider. They then estimated the number of women of reproductive age within each block group and created the statistics in the report.

Bearak noted that the study looked at only those clinics that provided more than 400 abortions a year or that were affiliated with Planned Parenthood.

"The goal here was to identify publicly available providers," he said. Although "there are situations where a doctor may provide abortion in a neighborhood," not every woman living in the area would be aware of the doctor's decision to provide pregnancy termination services.

"It can be risky for doctors to publicly advertise they are providing these services," Bearak said.

Since they were not easily identified as abortion providers, such locations were not considered public points of access, he explained, noting that stigma is commonly seen as a key measure of access, though it was not directly studied in this report.

Bearak and his co-authors found that half of American women of reproductive age live within 11 miles of an abortion provider.

"However, one in five would need to travel at least 43 miles," Bearak said, based on the report.

State by state, county by county

Distances to a clinic varied by state. In 23 states, the average distance was under 15 miles; for 16 states, the average was between 15 and 29 miles; and eight states averaged 30 to 89 miles. In three states -- Wyoming, North Dakota and South Dakota -- at least half of all women would have to drive 90 miles or more to reach the nearest clinic, the study found. These estimates are based on overall averages of the census blocks within each state.

Diving deeper into the data, the researchers found that in Alaska, one in five women would have to travel 154 miles or farther to get an abortion, while women in certain counties of Montana, Wyoming, North Dakota, South Dakota, Nebraska, Kansas and Texas would have to travel more than 180 miles.

For 34 states, though, the statistics appear largely unchanged between 2011 and 2014, and most changes in distances for other states were under 5 miles.

In some states, there was a decrease in the distance a woman would have to travel to reach an abortion clinic. Large changes were seen in Kansas and Maine -- decreases of 74 and 21 miles, respectively.

"I think if you lived in an area that previously had access to an abortion provider and suddenly you don't, it probably doesn't really help you that in another area, access improved," Bearak said.

Montana, North Dakota and Missouri saw increases in distance traveled: 46, 14 and 7 miles, respectively. The distance to clinics increased by at least 57 miles for one in five women in Texas and by at least 27 miles for one in five women in Missouri.

In particular, Texas, Missouri, Iowa and Montana had regions where the median distance to the nearest abortion provider increased, on average, by 30 miles or more between 2011 and 2014. Except for Iowa, each of these states adopted more restrictive abortion laws during this period.

Bearak said private doctors' offices as well as public health clinics could be providing abortion care, but the legal environment -- and stigma -- makes it very difficult for them to do so.

"These disparities don't need to exist," he said. "There are restrictions in place that have no basis in science, no basis in health, no basis in safety. These restrictions affect women directly, and they also affect health care providers directly."

Another option: telemedicine

Professor Ushma Upadhyay of the University of California, San Francisco, wrote in an accompanying editorial in The Lancet Public Health that "this study tells only part of the story."

Factor in the fact that some women must look for a provider who accepts Medicaid, say, or who is hospital-based, and the distance might be even greater, noted Upadhyay, who was not involved in the study.

"Research has shown that nurse practitioners and certified nurse midwives can safely provide medication and aspiration abortions without physician supervision," she wrote. These health providers might serve in areas with a lack of physicians.

Another approach could be telemedicine: In this case, a woman takes the abortion pill -- actually two medications, mifepristone and misoprostol -- while receiving real-time support from an online help desk overseen by doctors.

Though research indicates that telemedicine abortions have similar results to in-person abortions, Upadhyay noted that 19 states require abortion providers to be physically present with their patients.

Dr. Daniel Grossman, a professor at the UCSF School of Medicine, explained that when the Food and Drug Administration approved the abortion pill, it "applied kind of an extra layer of regulatory scrutiny to the drug," the kind usually applied to drugs with safety concerns. Yet today, "after 17 years of experience," the abortion pill has been found to be "very safe," said Grossman, who was not involved in the new study.

The heightened regulation means that the drug cannot be given in a prescription and dispensed at a pharmacy. It has to be handed out at a clinic, a doctor's office or a hospital, explained Grossman, so a doctor who needs or wants to prescribe a medication abortion has to arrange to stock the medication in her or his clinic.

This may not be possible for many doctors, especially those in rural locations.

With "lots of barriers to private physicians providing medication abortions," Grossman said, "that means, essentially, medication abortion is really only available at abortion clinics and not in doctors' offices." Access to this medication is more limited than need be, noted Grossman.

He is not alone in this belief.

The American Civil Liberties Union filed suit Tuesday in a district court in Hawaii to challenge FDA restrictions on where women can take the abortion pill. The lawsuit argues that these particular drug restrictions may be imposed only to ensure that a medication's benefits outweigh its risks. The abortion pill fails that test, according to the ACLU.

The FDA said it does not comment on possible, pending or ongoing litigation.

Greater distance, fewer abortions

"I think this study highlights -- and it's really turning into a situation where -- it really depends where a woman lives, whether she can exercise her constitutional right to an abortion," said Grossman, who has studied the issue of abortion access in the United States.

"In some places, access is relatively good, and the state may even provide financial help through state funds and Medicaid to help pay for the abortion, whereas women in other states may face many more barriers and obstacles to access care both geographic barriers and financial barriers," he said.

Overall, Guttmacher research indicates that abortions have declined by 14% since 2011. Although in some places, the decline may be "related to improvements in contraception use," Grossman said, in other places, the decline is "more related to barriers to access."

In a paper published this year in the Journal of the American Medical Association , Grossman and his co-authors showed how "distances were related to a decline in abortions at the county level," he said. "As the distance to the nearest facility increased, there was a clear relationship in the decline in the number of abortions."

If the distance increased between 25 and 49 miles, there was about a 25% decrease in abortion in a given area. As that change in distance topped 100 miles to the nearest clinic, there was a 50% decrease in the number of abortions in a given area, he noted.

"The other thing we found in our research in Texas was that these geographic barriers could also create delays for patients before they could access abortion care," Grossman said.

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"It takes time for women to arrange transportation, to gather the money together to pay for transportation. Sometimes, a woman has to decide to tell someone she wouldn't normally tell about the procedure in order to get that person to drive her to the clinic," he explained. "All these things take time, and this is obviously a time-sensitive procedure."

Greater distance to travel, then, meant some women had abortions in the second trimester.

"And a few women were unable to get the abortions they wanted," Grossman said.