IF Hollywood producers were asked to outline a plot guaranteed to boost ratings for a sitcom, they might title it "Modern Maternity." The sure-fire formula would go like this:Take a successful, unmarried career woman. Get her romantically involved with at least one man, preferably two. Then have her suspect she's pregnant - but leave the identity of the father a mystery, at least for a time. This is the story line that earned high ratings for the season finale of "Murphy Brown" last spring and kept viewers in suspense all summer. It's also the plot that propelled the show's season premiere to the top of CBS's charts last week as viewers tuned in to learn whether the father is Murphy's ex-husband, Jake, or her new boyfriend, Jerry. Jake won the paternity sweepstakes. Then, apparently underwhelmed by the prospect of becoming a father, he left to work in the Amazon, clearing the way for Murphy to embark on her own brand of career-woman chic - single parenthood. The 42-year-old Murphy, played by Candice Bergen, wrestled convincingly with questions that go to the heart of one of the most divisive issues of the day - abortion. Her decision in favor of motherhood promises a season sprinkled with warm and funny episodes on everything from prenatal tests and Lamaze classes to baby showers and childbirth. It is safe to predict that Murphy will look radiant in her designer maternity clothes. Her baby, swaddled in its nothing-but-the-best layette, will be adorable, even when cranky. As a working mother, Murphy will face challenges. But by the end of each 22-minute drama, cameras are sure to dissolve on a heartwarming scene, and audiences will smile approvingly. As a TV journalist trying to manage a career and a child, Murphy typifies the dilemmas of working women everywhere. Her executive producer summed up the show's appeal by telling the Los Angeles Times, "This woman is in every sense a role model." But even role models have limitations. However glamorous Murphy's solo venture into Modern Maternity appears on the TV screen, it bears little resemblance to the experiences of most real-life unwed mothers, for whom single parenthood is often more situation than comedy. More than one-fourth of all babies in the United States are born out of wedlock today, compared to only one-tenth in 1970. Many of these mothers are poor. More than half of all poor children in the US, in fact, live in female-headed house holds. Social mores have changed dramatically in the four decades since the very married Lucy and Ricky Ricardo produced the world's most famous sitcom baby, little Ricky, on "I Love Lucy." The on-air birth on Jan. 19, 1953, drew astronomical ratings and captured national headlines, even stealing some of the limelight from the Eisenhower inauguration. When the very unmarried Murphy gives birth next May, she may also capture the nation's heart - and probably the ratings. She will also reflect the new realities of tattered family life. Television, it goes without saying, is a powerful influence on manners and morals. What the Cyclops eye looks at without blinking, TV viewers will also tend to accept as the norm. If Murphy Brown plays out single motherhood as a kind of blithe encounter with the sitcom stork, a casual legitimacy will be granted to a difficult and lonely predicament that in real life jeopardizes two generations by its extraordinary demands on energy, patience, and money. It is a little ironic that at a time when cute babies with one parent missing are the darlings of TV and movie scripts, actors and actresses off-camera are taking parental responsibility more seriously. Earlier this year one veteran Hollywood trend-spotter told People magazine: "It's no longer chic to be unmarried and live a free life." The magazine noted that "new showbiz parents" like Michael J. Fox and Tracy Pollan, Kevin Bacon and Kyra Sedgwick, and Bruce Willis and Demi Moore "insist on a marriage c ertificate before ordering blueprints for a nursery." In the process of providing entertainment, sitcoms have an understandable tendency to make life seem easy - as effortless as a one-liner. But with a million-plus babies a year born out of wedlock, this might be one subject to keep off the laugh track.