My sister and I got together for a rare lunch recently and she asked a question that came to mind when I read about the rise in teen pregnancy in Montgomery County.

She wondered if I had talked "frankly and fully" about sex to my daughter, who is 16. The she added -- almost as if embarrassed - "You know we weren't told anything."

This appeal for openness on behalf of a favorite niece was a jolting reminder of the puritanical upbringing middle-class black girls received a generation ago.

It was a result of the general repressiveness of the era. But the puritanical attitudes were also transmitted by many black mothers, out to battle the stereotype that "all black girls are loose."

The black poet Toni Morrison captured this in a poem called "Mobile Respectability." She wrote, "They learn . . . the careful development of thrift patience, high morals and good manners. In short, how to get rid of the funkiness of passion, the funkiness of nature, the funkiness of the wide range of human emotions."

But things haven't changed enough since then. Despite more liberal attitudes about sex, most parents are as tongue-tied as ever about it. And that's particularly ironic for black parents since black women are the most likely teens to get pregnant.

A teen-age mother in Far Northeast recently told me that she "did not know anything about birth control" before her child's birth a year ago.

"I think somebody should have told me something," she said angrily. "I didn't want a baby. But I didn't want an abortion either."

Now her grandparents -- who were either unable or unwilling to talk to her about birth control -- are raising their grandchild and great-grandchild almost as siblings.

The high teen-age pregnancy rate flies in the face of a black society that tends to be more conservative in its personal and moral values (though not its politics) than the white society. (One local survey, for example, found that when it comes to religion and pornography, blacks think religion is more important than whites do and they feel more strongly than whites that pornography should be banned.)

This contradiction argues for more study, say the social scientists, who already know that racial and sexual discrimination and cultural poverty contribute to the high black fertility rate. So, far the first time, a number of experts around the country are probing the "why" of the black teen birthrate.

One factor is a continued wariness about birth control, which some blacks still view as an attempt by the white society to limit their numbers.

Others complain that professionals are not talking to black teens in their own language.

"Public policy has not been fair in dealing with the problem and what can be done," says June Dobbs Butts, a Washington psychologist and sex therapist. She conducted phone interviews with 10 leading black health professionals, who complained that black professionals were not called on to either plan or administer birth control services to black teen-agers.

In addition, they said that white professionals tend to impose their own values on the black teens they serve.

The National Council of Negro Women and the Department of Labor have just announced a pilot program of "employment models" to help teen mothers get into mainstream jobs.

It may be that the experts will bring greater understanding too late. Butts thinks that today's black family, which has traditionally made it possible for girls to keep their babies, is in trouble -- to often hard-pressed to provide food and shelter, much less emotional guidance, for teen-age parents.

But I evade my sister's question. Have I talked "fully and frankly"?

Truth to tell, sister, a bit of Louisville, Ky., still tinges my new Eastern edges. We've talked fully, but probably not often enough.

I've tried to explain morality to her broadly, as more than sex and spit kisses.

I've encouraged her to arrive at attitudes after carefully sorting ideas, values and experiences and, when she is ready, to develop a healthy, selective sexuality that involves a spiritual sharing.

I've argued against sex for high schoolers, for practical, not narrowly moral, reasons. Most of them aren't mature enough to understand the true meaning of sex are too scared to enjoy it, and run the risk of misadventures that will tarnish more mature sex later on.

And, oh yes, sister, I pray a bit.