Teen-age pregnancy, a social phenomenon intermixing ignorance, illness, illegitimacy and poverty, is costing the government billions of dollars each year in welfare support and medical outlays.

Some observers believe the cost will go up because of the ban on federal payments for most abortions.

Every year, 1 million teen-age girls, mostly single, become pregnant. Some are younger than 15.

From these pregnancies, there are about 600,000 live births a year. The others are terminated by miscarriages (100,000) and abortions were financed by the federal government until Congress clamped curbs on the use of U.S. funds for this purpose.

There are nearly 20 million teenage girls in the country, and slightly more than half are 15 or over, the sexually active category. In 1975, live births among the 15 to 19 age group totaled 582,238, with the under-15 group accounting for 12,642 live births.

The proportion of births to women in the 20 to 29 age category is much higher but that is to be expected because these are considered the normal child-baring years and a far larger proportion are married.

A large portion of the live births to teen-agers were to unmarried mothers, 233,500 in 1975. This exceeded the number of out-of-wedlock births to all other age groups combined. Virtually all unwed teen mothers (93 percent) keep their babies. More than half the out-of-wedlock teen-age births were to black girls

Kristin A. Moore of the Urban Institute recently calculated that in 1975, federal-state cash welfare payments to households containing teenage mothers, or older women who first gave birth in their teens, totaled $4.65 billion.

The figure, half the overall cost of the cash welfare program for families with dependent children, doesn't include the cost of food tamps, medical care and social services.

More conservative estimates of government outlays associated with teenage pregnancy range from $1 billion to $3 billion a year.

"Most of the problems we worry about are aggravated by teen-age pregnancy," said Peter Schuck, deputy assistant secretary of Health, Education and Welfare. "Unemployment, welfare, druge abuse" and the like.

Teen-age motherhood brings many complications. Many of the girls are poor or ignorant of medical needs and don't receive adequate prenatal doctoring or nutrition. Others, especially those in their ealy teens, ages 2 or 13 or so, are not physically fully grown and are less able to bear the trains of having a baby.

According to HEW figures, teen-age mothers have higher death rates, more hemorrhage, anemia, toxemia and lower-weight babies than women in their 20s. In many cases they don't finish high school. They find it hard to get jobs. Their babies are born with more birth defects than those of women in their 20s.

And many then slide into a life of overty, repeated pregnancy and welfare dependency. "Once a teen-ager comes pregnant and has a baby, it's likely to happen again," said Suhuck and Gerald Britten, another HEW deputy assistant secretary who is working on the problem.

They said 25 per cent of teen-age mothers get pregnant again within a year after their first baby; 15 percent within a second year. In statements before Congress, Secretary of Health, Education and Welfare Joseph A. Californo Jr. has repeatedly referred to this "repeater" tendency as condemning many girls to lives of poverty and welfare.

Not a great deal is known about teen-age pregnancy as a social phenomenon. But pioneering studies by John Kantner and Melvin Zelnick at Johns Hopkins School of Hygiene and Public Health, as well as ongoing statistical surveys by the U.S. government, have revealed the following:

In recent years, more teen-age girls have become sexually active and at an earlier age. In 1971, half of all unmarried black girls aged 15 to 19 and one-fifth of all white had had sexual intercourse. By 1976 the figure had jumped to 63 percent for black girls and 31 percent for white. The median age at time of first intercourse dropped to 16.2 years. The percentage of sexually experienced white girls, however, was rising much more rapidly than blacks.

By age 19, four-fifths of unmarried black teen-agers and half of white have had sexual intercourse.

The number of births to teen-agers in 1975 had fallen off slightly from 1966, from a bit over 600,000 to a bit under, but largely because of a decline in births among girls 18 to 19 - possibly because of legalization of abortion. The figures actually went up substantially for girls aged 10 to 14, particularly white. The birth rate among girls 15 to 17 rose sharply for several years but now is leveling down.

Although contraception reportedly was used regularly by about one-third of the sexually active teen-agers in a 1976 survey, the rest used in intermittently or not at all. About half of all sexually active teen-agers did not obtain contraceptive services from a doctor or clinic.

Among teen-agers, over half the episodes of first intercourse take place in the home of one of the partners.

Only 7 percent of unwed teen-agers who failed to use contraception, according to a study by Kantner and Zelnick, said they were trying to have a baby, but an additional 9 percent said they didn't use contraception because they didn't care if they became pregnant. Many of the balance said they thought they were "safe" because of the time of the month, the infrequency of intercourse or some other reason.

However, about a third said they avoided contraception for moral reasons or because it interfered with pleasure and spontaneity. This suggests, some believe, that even when contraception is universally available many will refuse to use it and will have unsought babies as a result.

There is a widespread theory among some groups that the availability of abortion and welfare is an inducement to girls to become pregnant or make little efforts to avoid it.

On the basis of various studies, Moore and Steven B. Caldwell wrote last year that the level of welfare benefits and acceptance rates "does not seem to serve as economic incentives to childbearing outside of marriage for either blacks or whites. In addition, the availability of contraception and abortion does not seem to encourage the individual to initiate sexual activity." This view, however, is far from universal acceptance.

One of the key questions in the teen-age pregnancy issue is why girls are becoming sexually active at an earlier age.

No one really knows for certain, but Lana Smith, director of Parent Focus, a project for developing school-based services for teen-age parents here, said one factor probably is earlier physical maturation of women, partly due to better nutrition.

She said the average age for the onset of physical maturity is now about 12 3/4 years, compared with about 16 1/2 years a century ago.

Other factors commonly cited, she said, are changes in sexual attitudes resulting from the women's movement, the so-called revolution in overall sexual attitudes of the 1960s, and the "heavy sexual message" coming across over radio, television and movies.

"There is a whole emphasis on macho for the man and physical sex for the woman," she said, nothing that one could commonly hear lines like "I want you to have my baby" and "Do you want to make love" in songs on radio.

Smith said she had come across many younsters in their early teens who failed to use contraception because they had a great deal of difficulty in full conceptualizing the consequences. They might understand the theoretical link between sex and pregnancy, but the probability that a baby might result for them was simply not a real concept.

This point of view was supported by Denese Shipp, director of the prenatal section of the Adolescent Pregnancy Program at the Johns Hopkins School of Medicine. Some youngsters, she said, say their first sexual encounter was "an accident." Shipp said social taboos against sex clearly don't seem to be as great as they once were.

Janet Forbush, acting deputy director of the forthcoming White House Conference on Families, also cited earlier physical maturation and the influence of the media.

Kantner, the Johns Hopkins social researcher, said he guessed the decline in the force of religious constraints, greater mobility, freedom in society (resulting in less "surveillance" of teen-agers than when parents were always together with the children), and earlier physical maturity were all factors.

He added, "Looking at this historically, whenever there is a period of social upheaval, and I think we're in one, this kind of thing" seems to increase.

In English history, for example, Kantner said there were several periods that witnessed a sharp rise in what used to be called "bastardy." Kantner said he doesn't think the availability of abortion and welfare is an inducement to pregnancy.

HEW is gearling up a new series of programs to promote contraception, family planning, sex education, and health treatment of teen-agers after they become pregnant. The bill is $142 million, of which $18 million is directly for more family planning services, and other portions will probably be used by community health centers and education programs for the same purposes. It is also proposing added Medicaid eligibility and research.

Richard Lincoln, an official of the Alan Guttmacher Institute, said his group and Planned Parenthood believe the HEW money input isn't enough, and at any rate may be underminded by administration support for continuing the ban on federal payments for most abortions that Congress mandated two years ago.

"The ban could lead to a new upsurge in births," he said. It isn't yet clear if this will happen, because many states are now paying for Medicaid abortions themselves, without federal help.