Those data are based on a nationally-representative survey of more than 7,600 women - including 288 who were trying to become pregnant - but don't provide an explanation for what may be causing the couples' infertility.

Researchers analyzed information from in-person and computer interviews conducted across the country in 2002 with women ages 15 through 44.

Germaine Buck Louis, from the National Institutes of Health in Rockville, Maryland, and her colleagues estimated infertility rates in two different ways.

First, they calculated the number of infertile couples as a fraction of all pairs that could or could not have become pregnant, based on their sexual behavior - resulting in a rate of seven percent.

Then they looked specifically at women trying to get pregnant, not including those who were using contraception or had very recently given birth, for example. That strategy showed 15 to 16 percent of couples couldn't get pregnant after at least a year of unprotected sex.

The finding is similar to smaller studies showing between 12 and 18 percent of women may have trouble getting pregnant, the researchers wrote in the journal Fertility and Sterility.

According to the Society for Assisted Reproductive Technology, women received close to 150,000 cycles of in vitro fertilization (IVF) in 2010, with male factor infertility and diminished ovarian reserve being the most frequent infertility-related diagnoses.

Dr. John Collins, a professor emeritus at McMaster University in Hamilton, Ontario, who has studied infertility, said there is a need for accurate measures of how widespread it is - but also that the rate may have risen in the last decade since this data was collected.

According to the Canadian census, he told Reuters Health in an email, the rate of infertility there rose from 8.5 percent in 1992 to 15 percent in 2009-2010.

Infertility specialist Dr. Sacha Krieg from the University of Kansas Medical Center in Kansas City agreed that infertility rates may be on the rise - possibly due to women waiting longer to try to have children or, more controversially, to the possible effects of environmental toxins.

"What this study showed, I felt, was a little bit higher infertility rate than we typically quote patients," Krieg, who wasn't involved in the new research, told Reuters Health.

"Many in the field feel like the rate of infertility is increasing, and (this finding) seems like a more accurate reflection of the actual infertility rate," she added.

Krieg said some couples who have been trying unsuccessfully to get pregnant for a while may still go on to conceive on their own, without the help of IVF.

Each cycle of IVF runs for about $15,000, and may or may not be covered by insurance.

Krieg recommended that people get checked out after a year of trying - or six months, for women over 35 - to make sure there aren't underlying problems, such as a blocked fallopian tube or low sperm count, preventing them from conceiving.

SOURCE: bit.ly/13m4aUt Fertility and Sterility, online January 4, 2013.

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