WHEN AN unmarried teen-age girl has a baby, she sharply reduces her own chances of ever leading a self-sufficient and successful life. Worse, her child enters the world with a drastically diminished prospect of getting the education and the guidance of other kinds that he will need to become a competent and productive adult. When early motherhood outside marriage becomes the rule rather than the exception in one distinctive part of the population, poor and black, that is a compelling reason for national concern.

In six remarkable articles in this newspaper over the past week, reporter Leon Dash described in detail the lives of some of these young mothers. In several cases he was able to trace the pattern of unmarried motherhood and misfortune back three generations. Mr. Dash lived for a year in Washington Highlands, a bitterly poor neighborhood on the southeastern edge of this city, where the birth rate among adolescent girls is extremely high. He talked at great length with the young mothers and fathers, and with their parents as well. It is striking that in these conversations any hope for a different future remains remote and insubstantial.

None of these characteristics is limited to blacks. Most of the babies born to unmarried teen-age mothers in this country are white. The birth rate among unmarried white teen-agers, although relatively low, has been steadily rising, while the birth rate among unmarried teen-age blacks peaked in 1971 and has been slowly falling since then. But it is the continuing very high rate among, specifically, young and poor black women that is particularly troubling.

Because the birth rate has been falling much faster among married black women than among the unmarried, the proportion of black children born out of wedlock has been increasing. Nationwide, three out of every five black children are now born to unmarried parents. Nearly half of all black children live in poverty. Two-thirds of all black children in female-headed families, with no husband present, live in poverty.

Still, it is also worth noting that among them, in Washington Highlands as everywhere else, there are girls who do not fall into early pregnancy and continuing dependency. They stay in school, graduate, and go adequately prepared into adult life. A good deal could be learned from these exceptional girls, who are growing up normally amidst abnormal and threatening surroundings.

But for the young mothers about whom Mr. Dash wrote, and all the others following their path, what can usefully be done? In this case it's hard to put full faith in conventional government programs and official attempts at intervention. The necessary changes have to take place at an altogether different level, in national attitudes and deep in people's convictions regarding personal morality. That is an area into which governments, not to mention newspapers, ought not to intrude lightly.

But it is true that over the years this country has inclined toward the view that people's conduct, including their sexual behavior, is not of concern to anyone but themselves. That proposition has had a terrible aggravating effect in Washington Highlands and the neighborhoods like it in which, amidst deep poverty, family structure is often fragile to begin with. Any real remedy has to begin with a willingness to see clearly and unsentimentally what has gone wrong, and at what cost to these young women and their children.