ATLANTA (AP) — American couples are not as infertile as it might seem.

Despite a boom in the use of fertility treatments, a new government study shows the percentage of married couples having trouble conceiving has actually dropped slightly in recent years.

About 6 percent of married women under 45 failed to get pregnant after at least a year of sex without contraception, according to the report. That's down from less than 9 percent some three decades ago.

That may seem surprising given how many women seek help to get pregnant; the use of fertility treatments, including in vitro fertilization, has doubled in the last decade. But that increase is driven by larger numbers of women trying to have children in their 30s and 40s, when female fertility declines.

So while infertility clinics may be more common and used more — particularly by more affluent women trying to have their first child at older ages — that's driven by a change in the market, not biology, said Anjani Chandra, lead author of the study.

"This runs counter to what a lot of people think," said Chandra, a researcher at the Centers for Disease Control and Prevention.

The study released Wednesday focused on married women. It was based on in-person interviews of more than 12,000 women and more than 10,000 men, ages 15 to 44.

The researchers did not ask why the couple was infertile — whether there was a problem with the woman's fertility or with the man's sperm. They did not include pregnancies that ended with miscarriage or stillbirth.

The results of the interviews conducted in 2006 through 2010 were compared to four similar surveys done since 1982. The trend was relatively flat, but overall, there was a slight decline.

"Despite years of doing these reports, people still think infertility has increased," Chandra said.

The researchers don't know what's behind the dip, but at least part of the reason may be a shift in how many women were trying to get pregnant. Since 1982, the proportion of couples who used contraception grew from less than 53 percent to more than 57 percent.

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The study did not find any significant infertility difference between white, black and Hispanic women. Nor was there any difference by education or household income.

Infertility was more common in women in their late 30s and early 40s, but that hasn't changed over the last 20 to 30 years, Chandra said.

What did change is there are more options for women who want to get pregnant and aren't conceiving. In the United States, fertility treatments, including in vitro fertilization, became generally available in the 1980s, and have become very common in the last 10 years.

"I don't think people wait as long" as they did decades ago to get help if they're having trouble conceiving, said Dr. Jessica Spencer, a fertility specialist with Emory University in Atlanta.

Another expert agreed with the CDC study's conclusion.

Today, "people are free to try to have babies when they want to, but the biology doesn't change," said Dr. Godfrey Oakley Jr., an Emory epidemiologist who studies birth defects.

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Online:

CDC report: http://www.cdc.gov/nchs/