The Soviet Union has changed its military draft law to open the way for women to serve in the armed forces.

The amendment was decreed by the Presidium of the Supreme Soviet on March 18 and was published in its official bulletin Friday. It appeared to reflect growing concerns here over declining population growth, which is likely to make it increasingly difficult to maintain the 4.8 million current strength of the armed forces later in the decade.

The new decree provides for draft registration of women with "medical and other specialized training." Such women between the ages of 19 and 40 could be required to undergo military training and be "accepted on a voluntary basis into active military service."

An American demographer, Murray Feshbach, in a report on the Soviet Union's population trends published two years ago, had forecast an acute labor shortage as well as a decline in the number of draftees in the second part of the 1980s.

The problem is that not enough children have been born to keep the economy running and to provide new recruits for the armed forces.

According to some western estimates, the Soviet Union would need to recruit 85 percent of all 18-year-old males by 1987 to maintain its forces at the present level. An alternative would be to lengthen the period of conscription beyond the present two years -- an unpopular move.

Military service in the Soviet Union is compulsory. All Soviet men, except invalids, are required to serve two years in the armed forces after leaving high school.

In previous years, college students could defer their service until graduation. But in an effort to cope with the problem, the Soviet military has drastically cut down on the number of deferments for educational and other reasons.

The change in the draft law was interpreted by observers here as a way to relieve pressures and to meet quota requirements by using women in communication, clerical and other jobs.

The demographic crisis also is believed to be one of the main elements in efforts to revive the Soviet economy by improving the productivity of dwindling manpower reserves. The late Soviet leader, Yuri Andropov, sought to cope with the problem by demanding greater social and labor discipline, strict punishments for absenteeism and drunkenness and a series of measures making it far more difficult to change jobs.

The new Soviet leader, Mikhail Gorbachev, appears to be moving in the same direction. However, drastic changes in the economy would be required to resolve the problem.

According to Soviet figures, the annual increment to the able-bodied age groups in the 1980s is less than half what it was during the previous decade. In the constituent republic of Russia, where most of the country's industry is concentrated, the population may actually fall by the end of this decade. Only in the Moslem southern parts of the Soviet Union -- still largely rural and unindustrialized -- will there be a net increase.

According to Feshbach, the ethnic Russians, who still compose more than 50 percent of the Soviet population, would cease to be the national majority before the end of the century.

According to his projections, net gains to the working age population, which averaged 2.7 million a year in the last decade, would shrink to about 300,000 a year in the 1980s, with most gains coming from the Moslem republics.

It is unclear whether the military would make special efforts to induce women to join.

A Soviet soldier receives 5 rubles a month (a little over $4 at the official rate of exchange) during his two years in service, an amount that allows little scope for recreation. The authorities discourage parents from sending money because it often is spent on alcohol.

The armed forces employ women, but they are paid regular civilian salaries. The new legislation seems designed to put more women in important jobs such as communications and computers and thus offset the influx of non-Russian speakers from less developed republics.