The Washington Post Co. reported a 12.7 percent decline in second-quarter profits yesterday and predicted its earnings this year will fall below 1985's results.

"The lackluster economy now forecast for the second half of the year will probably have an adverse impact on Newsweek advertising revenues," the communications company said. "As a result, it appears unlikely the company will surpass 1985's pro forma earnings per share excluding certain gains , after absorbing the dilution from the Capitol Cities cable acquisition."

The Post company reported revenue for the second quarter of $313.3 million and profits of $30.4 million ($2.37 a share) versus $284.3 million and $34.8 million ($2.66) in the same period last year. For the first six months, the company earned $42.8 million ($3.33) on revenue of $590 million, compared with profits of $59.7 million ($4.41) on revenue of $527.9 million in the first half of 1985.

The company said the 1986 results included a gain of $1.9 million (15 cents) from the adoption of a pension accounting rule. The 1985 first-half results included a gain of $12.2 million (90 cents) from the sale of various cellular telephone and SportsChannel interests.

Newspaper division revenue increased 5 percent in the second quarter and 4 percent in the first half of 1986 compared with the same period last year. Washington Post advertising rose to 1.423 million inches in the second quarter from 1.414 million inches, but for the first half was down to 2.682 million from 2.686 million last year.

Newsweek's revenue declined 3 percent in the second quarter and domestic advertising decreased to 634 pages from 722 pages last year. In the first six months of 1986, domestic advertising revenue rose 1 percent, while advertising pages fell to 1,192 from 1,272 in 1985.

Post-Newsweek television stations' revenue was up 3 percent in the second quarter and 5 percent in the first half of 1986, while the new cable television properties contributed $20 million in revenue in the second quarter and $40 million in the first half of 1986.

Earnings from affiliated companies, which amounted to only $81,000 in the second quarter last year, increased to $2.7 million this year and for the first half totaled $4.7 million, versus a loss of $67,000 in the first half of 1985. The gain was attributed to improved results from the company's newsprint operations and lower SportsChannel losses.