SHANGHAI -- It has been six years since I have been here. Much has changed. Tourists pour off cruise ships now and disperse in 20-bus convoys. Here and there stand new office buildings and hotels that would look fine on M Street -- if somewhat out of place in this turn-of-the-century downtown.

I marvel at these new Western brush strokes on the old Chinese canvas, as does any returnee. But something troubles me. I cannot see the Shanghai sidewalks anymore. They are covered with people.

It is difficult to escape the conclusion that the men now running China have lost their nerve in the struggle against inexorable, potentially disastrous population growth. Without ever saying so, they are now simply trying to stay ahead of it.

Six years ago I could walk down Nanking Road and not feel like a salmon on a Columbia River spawning run. There was still some space on the sidewalks then -- not much, but enough to allow brief stops for window shopping without being bumped or stepped on every two seconds.

By sheer force of numbers, rather than disdain for traffic rules, the pedestrians have spilled off the sidewalks and taken over the street lane closest to the curb. In some areas the authorities have bowed to the inevitable and erected metal railings in the middle of the street to bar automobiles from the curb lane and limit serious injuries. Unfortunately there are also more cars and trucks now. They must poke along in the jammed spaces of the drastically narrowed streets.

From one point of view, this means that the Chinese are doing very well. Our closest friends here, college-educated city workers my wife and I knew when we lived in China, have all acquired refrigerators. Many have secured bigger apartments. The latest campaign against "bourgeois ideas" seems to limit only public discourse. Private talk remains quite lively. The better-connected Chinese, such as one politburo family that welcomed us, dare to make all kinds of comments and contacts considered politically suspect.

So why fret about crowded sidewalks? The birth-control issue, after all, has lost the sense of urgency. The "One Child Is Best" billboard I used to pass every day in Beijing has been replaced by an appeal for health consciousness. In the foreign press, more Chinese babies long ceased to be front-page news. It may take a bad rice crop or two to remind us why we should worry about this. All the other interesting changes in China these days push the story into the background.

Publicity has never been essential to reproduction. The Chinese population passed the 1 billion mark in late 1981, a year after I left. It has added another 60 million people since, the equivalent of twice the population of California in just five years.

That growth has not gone unnoticed in Beijing. Judith Banister, China branch chief at the U.S. Census Bureau's Center for International Research, has followed Chinese birth-control policies for years and charted the bureaucratic meanderings and shifting targets behind each year's statistics. A new official Chinese 600,000-sample annual survey, plus some computer extrapolations, provide the best data we have ever had on Chinese growth.

After hovering around 2.1 percent, the Chinese birthrate dropped to 1.76 percent in 1980 with the advent of the one-child-per-family policy. A lower minimum marriage age allowed the rate to return to 2.1 percent in 1981 and 1982; more intense application of the one-child policy pushed it down again to 1.86 percent in 1983.

At this point reports of forced abortions -- of even near-full-term babies -- and the murder of female infants because they were undesirable reached the West. The birthrate dropped to 1.75 percent in 1984 and was 1.78 percent in 1985 but at a high cost.

In late 1984 the Chinese press began to signal a change. Banister and other Western researchers learned that a still unpublished "Document No. 7" had been issued in Beijing in April, softening the program. An appeal for "fair and reasonable" population control, clearly a phrase taken from the document, appeared in several Chinese newspapers. Given a choice between more population growth and more popular discontent with the Communist Party, the politburo took the lesser of two evils, and the results quickly showed.

Newly released official survey figures show the birthrate up to 2.08 percent, with the death rate, as usual, hovering around .6 percent to .7 percent. In a country where the vast majority of rural elderly can depend only on their sons for support, boy babies are preferred. In some provinces rural couples who produce a girl first are now allowed a second child. The old rules that kept unemployed peasants out of the packed but bustling cities have also eased. "Floating" populations of new city workers, not exactly legal, not exactly illegal, add to the midday crowds. Carl Haub of the Population Reference Bureau notes that the number of child-bearing-age women is about to take a significant jump, pushing the birthrate even higher.

Some dissident Western scholars, such as Thomas Sowell of the Hoover Institution, needle those of us who worry about overpopulation. They note that living standards in Hong Kong (14,322 persons per square mile) are better than in Chad (10.5 persons per square mile) and suggest that other factors, such as family traditions or the economic system, may make the real difference.

It is an interesting argument, worthy of study. Like it or not, the Chinese appear doomed to put it to a massive test, while the rest of us look on with interest, concern and some fear.

The writer, former Beijing correspondent for The Post, is now West Coast bureau chief.