One of every four pregnancies throughout the world ends in abortion, the Population Crisis Committee estimated yesterday in a new study of world abortion trends.

The study also estimated that half of the abortions are illegal.

The international population study group said there at least 40 million abortions annually, and that in some countries, the abortion rate far surpasses one in four.

"More than half of all pregnancies in Austria, Italy, Japan, Portugal, the Soviet Union and Uruguay are thought to end in abortion, either legal or illegal, [because] the most effective contraceptive methods - the pill, IUDs and voluntary sterilization - are not generally available in these countries," the report said.

For the United States, the abortion rate is relatively low, about the same one-in-four as the world average, because contraceptive methods are wide-spread, a committee official said.

In more than 60 countries, which have three-quarters of the world's population, abortion generally is legal. This includes the United States. In 30 countries, it is permitted under life-threatening circumstances only, and in another nine it is illegal - including Belgium, Indonesia, Egypt and the Philippines, the report said.

The report said that in the United States and Western Europe, women obtaining abortions "tend to be in their 20s or younger and are often seeking to postpone a first birth or avoid an out-of-wedlock birth." For example, the report said 74 percent of the U.S. women seeking abortion in 1976 were unmarried.

By contrast, in other parts of the world women getting abortions tend to be in their 30s or older, married and already have as many children as they want. "Many women in developing countries want no more children but lace access to family planning services and therefore resort to abortion, legal or illegal, to avoid an unwanted pregnancy," the report said.

Estimating that half of all abortions are illegal, the report called illegal abortion far more dangerous then legal abortion. In the United States, illegal, nonmedical abortions were estimated as 17 to 50 times more likely to result in deaths, with the risk climbing 25-to-250 times in developing nations.