ALEXEI, barely a month old, came straight from a ''maternity house'' in downtown Moscow to an adoring father and a mother who had trouble making his diapers fit. Though he didn't know it, Alexei was helping to make history.
The birthrate in the Soviet Union appears to be on the increase for the first time in four decades. If the trend is sustained, it could have major implications for this country's economy and its politics.
Even so, the effect will not be felt in the country's labor force for two decades. And in the interim, Western analysts say, the nation will be saddled with a labor shortage that will force some major changes in its economic planning.
It remains to be seen whether the upsurge in the birthrate will be significant and sustained enough to help the country avert even greater labor shortages and economic upheavals in coming decades.
Still, the fact that Soviet demographers have charted an uptick in the birthrate was enough to rate half-inch-high headlines in Pravda, the official Soviet Communist Party newspaper.
In 1970, the article noted, the birthrate was 17.4 per 1,000 population and just over 4.2 million babies were born. In 1981, the figures had edged upward to 18.5 percent and 5 million. In 1983, the birthrate stood at 20.1 per 1,000. That is higher, though not markedly so, than the United States.
But why are the figures important?
Because they obviously presage the numbers of workers entering the Soviet labor force, and that has far-reaching implications for this country's centrally planned economy.
The classic Soviet formula for economic growth, says a Western analyst, has been to increase the size of the labor force and the supply of raw material to produce more.
''The way they've grown in the whole post-war period is (primarily) because of increases in the labor force,'' he adds.
But because of the static birthrate over recent years, the Soviets are being forced to abandon that formula.
Abel Gezevich Aganbegan, director of the Economic Institute of the Siberian branch of the Soviet Academy of Sciences and one of this country's leading economists, has sounded warnings about the scope of the problem.
The Soviet labor shortage in coming years, he wrote in the trade union newspaper Trud, will be ''enormous.''
During the last five-year planning period (1976-80), he wrote, the labor force increased by 11 million people. During the current one (1981-85), the increase will be only 3 million.
''For the first time in our history,'' he wrote, ''we face the need to ensure production growth purely by increasing labor efficiency.''
Mr. Aganbegan also referred, in passing, to another aspect of the problem: Of the 3 million new workers coming into the economy during the current five-year period, 2.5 million will be natives of Soviet Central Asia and Azerbaijan, in the predominantly Muslim south.
That fans the fears of some of this country's leaders that Islamic fundamentalism and nationalism might take root in the region and challenge Slavic dominance in the USSR.
''The birthrate in the Central Asia just dwarfs the birthrate in the European parts of the Soviet Union,'' says a Western diplomat.
In 1983, for example, the Russian birthrate stood at 17.6 - up slightly from the year before. But in Uzbekistan, in the south, the birthrate was pegged at 35 .3 percent, almost twice as high.
Yet another aspect of the problem is the Soviet construction industry, which is notorious for delaying completion of projects it undertakes.
One Western analyst says, however, that this ''dark cloud may have a silver lining.
''The dark cloud is that this place can be terribly inefficient.
''The silver lining is that if they can just become a little more efficient, they still have room to grow.''
The Soviet leadership is fond of citing examples of factories that have actually increased output while decreasing the size of the labor force. And it continually beseeches factory managers to incorporate more high-technology and labor-saving devices into production processes.
But some zealous planners are arguing for even more strenuous efforts.
One academic proposed in Pravda the establishment of ''baby plans,'' whereby a government agency be made responsible for birthrates the same way other agencies are responsible for industrial production rates.
Only in this way, he argued, can ''certain unfavorable (demographic) trends'' be reversed.
Taking note of the slight increase in the birthrate, he said, ''By the end of the century, these children will have become workers, scientists, and defenders of the motherland.''
Some, however, have even more grandiose (and, some would say, darker) visions for matching future workers to society's needs.
A member of the Soviet Academy of Sciences, interviewed in Komsomolskaya Pravda, the newspaper of the Young Communist League, proposed a system of grading children at birth according to their potential abilities and treating them accordingly.
''I consider it necessary to determine a baby's capabilities immediately after it is born, calculate its grade, and according to this, program all his future development,'' said Prof. Vyacheslav Tabolin.
Of course, young Alexei, still in the cradle, may well be spared that fate - if there are enough other youngsters born to fill future jobs and allow continued expansion of the economy.