LOS ANGELES (AP) _ NBC won the first week of the May sweeps ratings contest with help from an unrelenting advertising campaign for its ripped-from-the-headlines miniseries ''Switched at Birth.''

Part one of a true-life story about two Florida couples whose newborns were accidentally switched in a hospital nursery was the most watched show on television last week, according to figures released Tuesday by the A.C. Nielsen Co.

NBC, which aired scores of radio and television ads trumpeting the program, also won the overall network ratings race with a 12.7 average. ABC was second with 11.4 and CBS was third with 11.1.

Each ratings point represents 931,000 homes.

Last week marked the beginning of May sweeps, a monthlong period during which networks schedule blockbuster programming to boost ratings. Ratings during sweeps periods determine advertising rates for affiliate stations.

NBC's ''Cheers'' was No. 2 for the week, followed by CBS' Sunday news magazine ''60 Minutes.''

An episode showing series stars Susan Dey, Harry Hamlin and Jimmy Smits leaving the fold of ''L.A. Law'' pushed the series into a two-way tie for fourth place with CBS' ''Murder, She Wrote.''

The trio from television's favorite yuppie law firm had said they were leaving the show to pursue other projects. But Dey announced Tuesday she was returning next season. Whether or not Hamlin and Smits will return in guest appearances was not announced.

A rerun of ABC's hit comedy ''Roseanne'' was No. 6.

The premiere of ABC's ''Dinosaurs'' tied for seventh place with NBC's reality-based whodunit series ''Unsolved Mysteries.''

ABC's ''America's Funniest Home Videos'' was No. 9.

NBC's broadcast of ''The 26th Annual Country Music Awards,'' featuring weepy performances by The Judds and Reba McEntire, and ABC's ''Family Matters'' tied for 10th place.

ABC and Peter Jennings continued to dominate the network news race, finishing last week with a 9.5 average. NBC and Tom Brokaw were second with an 8 and CBS and Dan Rather were third with 7.9.

Here are the top 10 prime-time shows, their network and rating: ''NBC Sunday Movie: Switched at Birth,'' 20.4; ''Cheers,'' NBC, 19.3; ''60 Minutes,''CBS, 17.1; ''L.A. Law,'' NBC, and ''Murder, She Wrote,'' CBS (tie) 17; ''Roseanne,'' ABC, 16.2; ''Dinosaurs,'' ABC, and ''Unsolved Mysteries,'' NBC, (tie) 15.9; ''America's Funniest Home Videos,'' ABC, 15.8; ''The 26th Annual Country Music Awards,'' NBC, and ''Family Matters,'' ABC, (tie) 15.6.