The popularity of artificial insemination by donor and other surrogate procedures might suggest that infertility is on the rise in the United States. But because of changes in when people are trying to become pregnant and in attitudes toward infertility, it is difficult to pin down a trend.

Physicians define infertility as the inability to conceive after trying for at least a year. "Primary infertility" refers to childless couples; "secondary infertility" to couples who have had at least one child. In both cases, government studies have dealt with married couples in which the wife was between 15 and 44 years old.

Between 1965 and 1982, primary infertility in married couples has doubled, according to the National Center for Health Statistics, but secondary infertility has gone down sharply.

The National Survey of Family Growth in 1982 found 2.4 million couples (roughly one in 12 of all American married couples in that age bracket) to be infertile -- 1 million with primary infertility and 1.4 million secondary.

In 1965, the numbers were 3 million total -- 500,000 primary and 2.5 million secondary.

In general, the studies found infertility more common among older women (those over 35) of reproductive age than younger ones, and higher in blacks than in whites. With more women waiting until later in life to have their first child, this may account for some of the rise in primary infertility.

But the situation was actually more complex.

For example, the statistics revealed increased infertility among one group of younger women: those 20-24. The rise in such sexually transmitted diseases as gonorrhea and chlamydia -- both of which can render women sterile -- was, infertility specialists believe, largely responsible for this trend.

And although the 1982 survey found older black women to be at highest risk of infertility, it also found that young white women were far more likely to have sought medical attention for it.

NCHS statistician William Mosher suggests that the rise of surgical sterilization -- vasectomy in men and tubal ligation in women -- may account for much of the drop in secondary infertility between 1965 and 1982. "If a couple is surgically sterilized, they obviously won't discover later that they are infertile," he said in an interview.

Sociologist Sevgi Aral at the Centers for Disease Control in Atlanta notes two factors that have made infertility treatment increasingly popular: (1) technology as a driving force for social change and (2) Parkinson's Law, which holds that work expands to fill the time available for it.

"The attitude used to be that 'if I can't have a baby,' that's the way it was," Aral said in an interview. "Now, what with new technology and all the attention that has been focused on career women needing to become pregnant before it is too late, both behavior and expectations have changed."

At the same time, Aral noted, obstetricians found themselves less busy as the baby boom ended, making infertility services a promising new field of activity for these specialists.

The implication is that economics is at work here, with those women who are best equipped financially to seek help going after it, while poorer women with fertility problems remain untreated. The idea is the more plausible because few health insurance plans offer coverage for infertility services, and these services do not come cheap.