A quarter of the 6.1 million pregnancies in the United States in 1981 ended in abortion, according to the latest report by the Alan Guttmacher Institute, a research organization affiliated with Planned Parenthood. That's 1.5 million abortions. Much remains to be done to reduce unwanted pregnancies.

White women, who comprise the largest racial group in the nation, accounted for 70 percent of the abortions, or 24 per 1,000 pregnancies. The rate for nonwhite women, however, was more than double that: 56 per 1,000 pregnancies. Females 15 to 19 obtained nearly half a million abortions.

The Children's Defense Fund, which launched a project to prevent teen-age pregnancies in 1983, recently issued data that show the dimensions of the problem in that age group: Teen-agers are responsible for more than half a million births every year, 10,000 to children under the age of 15.

"Almost 90 percent of the pregnancies among unwed teens are unintended," the CDF found. "For the over 800,000 teens who faced unintended pregnancies, there were no satisfactory choices or outcomes; 60 percent aborted their pregnancies; virtually all of the remaining 40 percent chose to raise their children themselves, half as single parents.

"The consequences of teen-age pregnancy go beyond personal tragedy. Society pays as well. Over half the mothers on welfare had their first child in adolescence. Thirty percent of all hospital deliveries to adolescents are paid for by Medicaid. Because teen mothers are the least likely to receive prenatal care, their babies are most likely to need expensive hospital care after they are born. Babies born to teens account for 20 percent of all low birthweight infants born each year."

These statistics cry out for a targeted approach to family planning that would reach teen-agers, particularly nonwhites, who already have higher poverty rates and are going to be the most vulnerable to the crippling effects of adolescent motherhood. Their fate could not be more bleak or predictable: Young women who cannot complete their educations cannot get decent jobs that pay enough for child care and cannot support themselves, much less a child. Their futures are behind them.

The CDF report found that while 14 percent of the adolescent population is black, black women account for 28 percent of births by adolescents, and nearly half of all births to unmarried teen-agers. "Eighty-five percent of black single mothers under age 25 live in poverty," the CDF report noted. It said the pregnancy rate among young black women "is a crisis that threatens to cripple economic progress and lock generations of children into poverty."

The CDF report said that the extended family, a support system that used to help young mothers, has changed since the early '50s. The black extended family "today is likely to be a 17-year-old mother and 35-year-old grandmother each with one minimum wage job, if any. The community has lost many of its leaders to the suburbs and now, deplete of role models, confirms rather than counters black youths' fears that life holds little for them. Most important, the adolescent single mother was the exception in the black community of the 1950s. Today, she is the rule."

The abortion and teen-age birth statistics both underscore the need for more and better birth control information for young people. While the burden of pregnancy falls most heavily on young women, whether they give birth or have abortions, both young men and young women need to understand the devastating effects of premature childbearing on young women and their children.

But that is only part of the answer. The CDF report makes the point that teen-agers who believe they have opportunities and a bright future ahead of them are more likely to be responsible about sex and birth control. This is the very group, however, that faces the highest rate of unemployment.

Society can continue with a half-baked approach to teaching sex education, and it can continue with its half-baked approach to poverty and minority unemployment. But the result will be a perpetually costly group of mothers who were trapped into poverty as teen-agers and a perpetually high rate of abortions. Or society and its institutions can tackle the causes of teen-age pregnancy by making more and better contraceptive information available and by providing its young people with education, skills and opportunities.

As things stand, society is spending hundreds of millions in various forms of minimal assistance and young women, particularly young black women, are paying with their futures.