The Washington Post Co. reported yesterday that earnings for the first quarter of 1978 increased 41 percent over the same period a year earlier, from $5 million (57 cents per share) to $7.1 million (86 cents per share.)

Revenues for the quarter were $112.8 million, up 19 percent over the $94.6 million of the first quarter of 1977.

The quarterly results include operations of the Everett (Washington) Herald since it was purchased on Feb. 14, but the company said the acquisition's contributions were not material compared to operations as a whole.

Revenues of the company's newspaper division grew by 17 percent, without counting the Everett Herald. Advertising lineage was up 4.6 percent at the Washington Post and 8.9 percent at the Trenton (N.J.) Times.

The magazine and book division reported a 22 percent increase in revenues. Advertising income increased 27 percent, as the number of advertising pages in Newsweek's domestic edition grew from 575 to 641. Circulation revenues were up 16 percent, because of higher subscription prices.

The broadcasting division had an 11 percent increase in television revenues. The company has agreed to sell its only radio station (WTOP).

Dynalectron Corp. of McLean said yesterday it lost $420,000 (7 cents per share) for the first quarter of 1978, more than twice its 202,000 (4 cents per share) first quarter loss last year.

The world-wide technical services company reported revenues for the quarter increased from $55.1 million last year to $66.9 million.

A Dynalectron spokesman blamed the bigger deficit on weather delays in construction and other work, increased costs in water treatment and electrical construction projects underway, and foreign currency translations, which amounted to about $154,000 of the loss.

Overnite Transport also said the bad winter hurt its business, but a 19 percent increase in tonnage enabled the Richmond truck line to boost first quarter net income by 27 percent, from $1.8 million (58 cents per share) to $2.3 million (73 cents per share).

Revenues for the period increased 29 percent, from $33.9 million to $43.9 million, and the company said recent rate increases will mean about $10 million in added revenues this year.

Fidelity American Bankshares Inc. of Lynchburg, said its assets reached the $1 billion mark in the first quarter, while its net income grew from $1.5 million to $1.6 million. Because of changes in the number of shares outstanding, per share income declined, from 60 cents to 59 cents.

Fidelity American, which has 15 member banks, reported operating income grew from $1.3 million to $1.6 million. The major improvement, a spokesman said, was due to a 14.5 percent rise in loan volume. Assets increased from $907 million to $1 billion.

Commonwealth Natural Resources of Richmond said its net for the first quarter fell from $2.6 million ($2.25 per share) to $2.3 million ($2.03 per share) Revenues increased from $37 million to $42 million for the distributer of propane and other gases.