Infertility is defined as the inability to conceive after one year of trying, or the inability to carry pregnancies to a live birth. It affects one out of six couples of childbearing age in the United States today - at least ten million people. And in a career-oriented area like Washington, where many couples postpone decisions about childbearing until professional goals have been met, the ration of one in six is probably on the conservative side. Yet it is rarely discussed, and understood even less. For almost all couples the condition comes as a surprise. And no wonder. It seems as if the whole is on its guard against producing unwanted children. Every day 19.9 million women in this country wake up and remind themselves to take the Pill. In China, a woman with more than three children is considered an enemy of the state. In India, population experts fear the country may end up at century's close with four times as many people as it started with - up from 250 million in 1900 to one billion. The huge nation has resorted to quickie vasectomies and cash rewards at commuter train stations. Two will do posters are everywhere. Although infertility may affect people of all social classes, the childless poor usually have neither the time nor the money to undergo a lengthy series of tests - commonly called an infertility work-up - to determine the cause of the problem. There may also be class differences in a person's willingness to endure many sacrifices so that a long-range goal can be realized. For these reasons, the inability to conceive and bear children seems to be a middle and upper middle-class problem. The anguish of infertility will strike increasing numbers of couples in the next few years, however, as the children of the baby boom reach their late twenties and early thirties. Many who till now have postponed marriage and childbearing for their careers will turn to both to round out their lives - and find child-bearing not possible. The men, after years of enjoying what they consider a healthy sex drive, will be shocked to learn that their sperm are too few in number or perhaps not active enough to effect a conception. The women may be given a finding of endometriosis, a condition in which parts of the uterine lining seed themselves in various places along the reproductive tract. Unknown in cultures where women marry young, it is a common finding in American women past 30. Or the women may be part of the 10.9 million who took the Pill every day whether or not previous gynecological abnormalities should have warned the physician against a prescription. A generation ago, before the current explosion in medical technology, many couples who could not produce children were told there was nothing wrong with them: either it was all in their heads or God's will. There was also smirking ignorance on the public, the insinuation they weren't performing correctly in bed. In fact, male impotence is the source of less than five per cent of the cases of male infertility, and the sources of impotence are extremely varied, from diabetes to perineal nerve injury to psychogenic causes.