"Guess Who's Pregnant?" is not what could be called an accomplished or resourceful piece of video journalism, but this public TV documentary about teen-agers and sex has such a heavy dose of potent information that the message pretty much carries the hour.

One could wish that more people would have a chance to see this show, which is aimed at parents but obviously concerns their kids as well. Channel 26 and other public TV stations will be showing the documentary, produced by WTTW in Chicago, at 10 o'clock tonight. But it might be made available to interested community groups if there is some sort of future demand.

Actually and ironically, there may be no demand, because one of the reasons here are an estimated 1 million new teen-age pregnancies in this country each year is that parents resist the idea of making adequate sex education or birth control devices available to their sexually active adolescents.

Producer-writers Michael Hirsh and Elayne Goldstein trot out the usual parade to make their points: exports, statistics, case studies. Some of their most pertinent film was shot in the Washington area, particularly at a model sex education program underway at George Mason High in Falls Church, Va.

While in the area, Hirsh and Goldstein took their crew to hearings on proposed sex education in Fairfax County schools, and they recorded some of the excuses parents give to promulgate ignorance in the young - chilling, incredible 19th-century stuff. It's obvious that against deep-rooted fundamentalist notions about sexual behavior, reformers stand little chance of lowering the teen-age pregnancy rate or wising up kids about the real wages of whoopee.

The program wastes considerable time on such digressions as a montage of sexual stimuli in the mass media, and a tasteless close-up of a baby slobbering was inserted right in the middle of a moving monologue by a teenage mother from Chicago.

And while the writers take commercial TV to task for refusing to air ads for condoms (a fairly funny sample spot is shown), the possibility of a sex education series on public TV is never discussed.

Still, "Guess Who's Pregnant?" ought to have considerable impact on those who see it, whether they are teen-agers, parents of teen-agers or merely bemused observers of the state of the world.