West Germany was the world's biggest exporter for the first three months of 1990, regaining its leadership over the United States, the International Monetary Fund reported yesterday.

West Germany increased its exports to $99.8 billion, up 18 percent over the first quarter of last year. The U.S. increase of nearly 10 percent brought its total to $97.2 billion.

In 1989, the United States regained first place with $364 billion worth of exports compared with West Germany's $341 billion. West Germany first moved ahead in 1986.

The unification of West Germany and East Germany could put the united Germany ahead of the United States as the world's biggest exporter for years to come, experts said. The IMF gave no figures for East Germany, which is not among its members.

Japan was third in exporting, with $274 billion for 1989 and $68 billion for the first quarter of this year.

The United States remained the world's biggest importer, with $125 billion in foreign-made goods coming into the country in the first quarter of the year, compared with $118 billion a year earlier.

West Germany had a $22 billion surplus of exports over imports for the quarter, compared with a U.S. deficit of $28 billion. Japan was the only one of the seven major industrial nations to report a decline in exports for the quarter. Japan still had a $12 billion surplus for the period, second only to West Germany.