The United States leads the developed world in sexual pietism. We are among the most skittish when it comes to talking about sex, teaching our children about sex or providing contraceptive devices and services for our teen-agers.

The United States also leads the developed world in teen-age births, abortion and pregnancy and is the only developed country in which teen-age pregnancy has been increasing in recent years.

No, it isn't because of the much-remarked increase in pregnancy among black teen-agers. Leave out blacks altogether, and the pregnancy rate for U.S. teen-agers remains significantly higher than the rates for England, France, Canada, Sweden and the Netherlands.

These recent findings of the Alan Guttmacher Institute (AGI) may not hold major surprises for professional family-planning practitioners, but they are sure to leave many of the rest of us unsettled -- for instance, those who believe there is a positive correlation between sexual sophistication (sex education, contraception counseling, etc.) and pregnancy; those who think liberal attitudes toward sex tend to increase teen-age pregnancy; those who imagine that enforced ignorance leads to chastity, and even those who see the whole problem in moral terms.

The AGI investigators believe that sexual activity in the five countries featured in the study is not very different from that in the United States. That is one of the reasons the five were selected (out of a total of 36 from which data were gathered). But while their sexual activity, economic development and cultural backgrounds are quite similar to those of the United States, their teen-age pregnancy rates are much lower.

In England and Wales, for instance, the pregnancy rate for girls aged 15 to 19 is 45 per thousand; for France, 43 per thousand; for Canada, Sweden and the Netherlands, the numbers are 44, 35, and 14 per thousand. The U.S. rate is 96 per thousand overall, 83 per thousand for whites only. Moreover, the greatest gap between the United States and the others is in the youngest cohort -- the 15-to 17-year-olds.

The study suggests that our sexual pietism (as reflected in such policies as the now-abandoned "squeal rule") may actually contribute to the problem. Those countries with the most liberal attitudes toward sex, the most easily available (and cheapest) contraceptive services and the most effective formal and informal sex-education programs have the lowest rates of adolescent pregnancy, abortion and childbearing.

Nor did the study uncover any evidence of a link between welfare benefits and teen-age childbearing or between teen-age job prospects and pregnancy.

The countries with the lowest adolescent pregnancy rates all have free (or virtually free) contraceptive services, with "the pill" accepted as the most appropriate method; all have free (or subsidized) abortion services for teen-agers, usually kept confidential even from the teen-agers' parents, and all appear to be more tolerant than the United States toLFward teen-age sex in general.

"American teen-agers," the study observes, "have inherited the worst of all possible worlds regarding their exposure to messages about sex. (The media) tell them that sex is romantic, exciting, titillating, yet at the same time (they) get the message that good girls should say no. Almost nothing they see or hear about sex informs them about contraception or the importance of avoiding pregnancy."

That may help to explain why the United States has the lowest level of contraceptive use among teen-agers in all six countries.

The AGI findings will be reassuring to the pragmatists for whom reducing adolescent pregnancy and childbearing is the major goal; much less so to those who see reduced adolescent fertility primarily as a byproduct of increased morality. For those whose notion is that premarital sex is simply wrong, the findings will be seen as helping youngsters avoid the consequences of sin.