NEW YORK – U.S. abortions fell 5 percent during the recession and its aftermath in the biggest one-year decrease in at least a decade, perhaps because women are more careful to use birth control when times are tough, researchers say.

The decline, detailed on Wednesday by the Centers for Disease Control and Prevention, came in 2009, the most recent year for which statistics are available. Both the number of abortions and the abortion rate dropped by the same percentage.

Some experts theorize that some women believed they couldn’t afford to get pregnant.

“They stick to straight and narrow and they are more careful about birth control,” said Elizabeth Ananat, a Duke University assistant professor of public policy and economics who has researched abortions.

While many states have aggressively restricted access to abortion, most of those laws were adopted in the past two years and are not believed to have played a role in the decline.

Abortions have been dropping slightly over much of the past decade. But before this latest report, they seemed to have pretty much leveled off.

While experts estimate there are more than 1 million abortions nationwide each year, the CDC counted about 785,000 in 2009 because of incomplete reporting.

To come up with reliable year-to-year comparisons, the CDC used the numbers from 43 states and two cities — those that have sent in data consistently for at least 10 years. The researchers found that abortions per 1,000 women of child-bearing age fell from about 16 in 2008 to roughly 15 in 2009. That translates to nearly 38,000 fewer abortions in one year.

Mississippi had the lowest abortion rate, at 4 per 1,000 women of child-bearing age. The state also had the nation’s highest teen birth rate. New York, second to California in number of abortion providers, had the highest abortion rate, roughly eight times Mississippi’s.