Ford Motor Corp. yesterday announced a 12.7 percent drop in first-quarter earnings, joining the nation's other Big Three auto makers in reporting a decline in profits.

Ford said its earnings fell to $783.3 million ($4.20 a share) from $897.2 million ($4.90) in the year-ago period.

The falloff in earnings occurred despite a 2 percent increase in the dollar value of worldwide sales of Ford products. The company reported first-quarter revenue of $13.2 billion versus $13 billion for the same period a year ago.

Ford's overall tax bill for the quarter amounted to $527 million, which "more than accounted for . . . the decline in this year's first-quarter net income," the company said in a statement issued yesterday.

"On a before-tax basis, profits were a record $1.3 billion, up $18 million from a year ago and the 16th consecutive quarterly improvement from the prior year," the Ford statement said.

Ford's after-tax profits in the United States were $626 million for the quarter, $57 million lower than profits in the comparable 1984 quarter.

Ford's overseas operations generated net gains of $157 million, compared with $214 million in the same period of 1984. "The reduction was more than explained by higher taxes," the company said.

Profits for the nation's three largest auto makers dropped 26.3 percent in the first quarter of 1985, falling to $2.36 billion from $3.20 billion earned in the same period last year.

Ford, General Motors Corp. and Chrysler Corp. all attributed the decline to new-product development costs and increased corporate income taxes.

GM, the nation's biggest car company, experienced the largest drop in Big Three first-quarter earnings. GM's net income for the quarter was $1.07 billion ($3.26 a share), a 33.6 decline from $1.61 billion ($5.11) in the same quarter last year.

Chrysler's earnings fell 28.1 percent, to $507.6 million ($4.18 a share) from $705.8 million ($5.64) it earned in the 1984 quarter.

The Big Three car companies are expected to spend about $15 billion this year to modernize plants, build new facilities and produce new cars. GM alone plans to spend about $9 billion on plants, equipment and tooling. Chrysler officials say their capital expenditures for 1985 will reach $2.8 billion, and Ford is investing $3 billion to bring out its new Ford Taurus and Mercury Sable passenger car lines this fall.

The Taurus and the Sable will mark Ford's second generation of super-aerodynamically designed cars, now represented by the Ford Tempo, Mercury Topaz, Ford Thunderbird and the Lincoln Continental Mark VII.