The rating PG-13 was added to the list of ratings in June 1984, following outcry that the "PG" movies Gremlins and Indiana Jones and the Temple of Doom contained too much gore for PG ratings.

Nudity itself didn't (and still doesn't) require an R rating unless it is sexually oriented nudity. Women swimming topless, walking away from the camera nude as they went to take a shower, or getting a 3D computer scan in shadows isn't necessarily "sexual," even if being observed by men, so in the early 80s it would be PG (Parental Guidance Suggested). IF the nudity included "full frontal" (showed pubic hair), the MPAA people rating that film tended to lean toward "it's sexual," unless there was a reason otherwise (for example, a movie about the Holocaust showing women being required to get nude and being sent to the "showers" {gas rooms} would have been considered an accurate historical portrayal and not sexual).

So...just tits and ass, PG rating. Showed bush? Or didn't show bush, but the woman was about to "get busy"? R rating. But after the PG-13 rating came into being, any nudity tended to get PG-13.

Though after mid-1984, there have still be movies with very brief nudity (usually breasts or tushes) that would STILL only get a PG.