The number of abortions reported to the Centers for Disease Control and Prevention fell by 2 percent year over year and by 24 percent over the span of a decade, a new report shows.

There were 638,169 abortions reported to the CDC in 2015, the most recent year for which data tallied by the agency are available, down from 652,639 in 2014 and 842,855 in 2006. The shifts may be tied to a decline in the share of pregnancies that are unintended and to the increased use of more effective contraceptives, the CDC said.

"Unintended pregnancy is the major contributor to induced abortion," the report says. "Increasing access to and use of effective contraception can reduce unintended pregnancies and further reduce the number of abortions performed in the United States."

After the landmark Supreme Court decision Roe v. Wade guaranteed abortion rights nationwide in 1973, the total number of reported abortions "increased rapidly, reaching the highest levels in the 1980s before decreasing at a slow yet steady pace," the report says.

Between 2006 and 2015, the rate of abortions per 1,000 women ages 15 to 44 fell from 15.9 to 11.8, while the ratio of abortions per 1,000 live births fell from 233 to 188, the CDC report says. Data were voluntarily reported by most states, the District of Columbia , and New York City. California , Maryland and New Hampshire did not consistently submit data and were excluded from trend analyses.

Notably, the CDC says those three states accounted for 20 percent of the estimated 926,200 abortions tallied for 2014 through a national census of abortion providers by the Guttmacher Institute, a reproductive rights research and policy organization. The CDC also says data from the District of Columbia and New Jersey are likely incomplete as medical providers in those areas are not legally required to submit abortion information to a health agency.

While restrictions enacted in some states during the last few years could make it more difficult for women to obtain abortions, Medicaid expansion through the Affordable Care Act may have made the procedure more accessible in other areas, according to the Guttmacher Institute.

Among women who were 15 to 44 years old in 2014, nearly 1 in 4 were projected to have an abortion by the time they turned 45 if age-specific rates that year held steady, according to a Guttmacher analysis published in the American Journal of Public Health in 2017. Yet falling abortion rates have not been consistent across socioeconomic and racial groups, CDC and Guttmacher data show.

"Throughout the years, the incidence of abortion has varied considerably across subpopulations and remains higher in certain demographic groups than others," the CDC report says.

In 2015, women in their 20s had the highest abortion rates, the CDC said, and among the 39 areas that reported marital status data, about 86 percent of women who obtained an abortion were unmarried.

Among the 30 areas that reported race and ethnicity data, white and black women accounted for about 73 percent of all abortions in 2015. Black women had an abortion rate of 25.1 abortions per 1,000 women 15 to 44 years old, while white women had a rate of 6.8 per 1,000 women, the CDC said.

Some data suggest the gaps in abortion totals, rates and ratios between black women and women of other groups "narrowed from 1994 to 2008, but remained steady from 2008 to 2014," according to the CDC.

In 2015, women with one or more previous live births accounted for about 6 in 10 abortions, and a "substantial proportion occurred among women with a previous induced abortion, events that are also opportunities for contraception counseling," the report says.

Nearly two-thirds of abortions in 2015 occurred within the first eight weeks of gestation, and 91.1 percent were performed within the first 13 weeks, the CDC said. Women who have abortions after the first trimester may do so because they must travel far distances for the procedure, face financial or legal limitations, or have concerns about fetal health, according to Planned Parenthood .

Understanding the differences in abortion rates is key to reducing disparities in reproductive health care, according to the Guttmacher Institute.