The Food and Drug Administration is preparing to warn pregnant women that they should stop or "minimize" drinking of coffee, tea and colas because the caffeine in these beverages may cause birth defects.

The FDA warning, it was learned yesterday, will be based on the agency's most recent reviews of animal tests in which rats fed caffeine produced offspring with abnormal numbers of defects.

At the same time, FDA Commissioner Jere Goyan is expected to say that the evidence against caffeine is not yet conclusive.

FDA officials have therefore decided not to propose a warning label for caffeine-containing products -- coffee, tea, colas, some other soft drinks, chocolate and some drugs. The possibility of a warning label had been under consideration for several months.

The agency will formally propose, however, that soft-drink makers be permitted to market decaffeinated versions of their products and still call them "colas." That is not permitted under current FDA regulations.

"What Dr. Goyan has decided," said an agency source, "is that there is not enough evidence against caffeine for regulatory action, but there is enough evidence to require him to tell women they need to be prudent about their caffeine intake during pregnancy. And giving them soft drinks without caffeine will give them some alternative."

Goyan is expected to give no scientifically precise definition of a "prudent" or "minimal" or "safe" level of caffein consumption during or just before pregnancy. No one knows what the "safe" level might be, said agency sources, though one may exist.

FDA officials generally do not think caffeine is causing any readily observable birth defect epidemic, even though it could be causing some unknown number of cases, if FDA's rat tests are accurate.

But FDA is doing further animal testing, in part because some studies indicate rats may metabolize caffeine differently from humans. The agency also expects to require the coffee industry to start long-range human studies of caffeine effects.

A Washington public advocacy group, the Center for Science in the Public Interest, petitioned FDA in November 1979 for warning labels on coffee and tea. Dr. Michael Jacobson, the director, maintained both animal and human studies have linked caffeine to birth defects that include cleft palates, heart defects and missing fingers and toes.

He too said no one knows whether there is a safe caffeine level, but some studies have indicated birth defects may appear in offspring of women who drink just four cups of coffee daily. He said "as many as 400,000 pregnant women a year" drink four cups daily, and "women who are or may become pregnant" should at least minimize consumption.

By federal figures, a cup of coffee contains 75 to 155 milligrams of caffeine; a cup of tea brewed three minutes up to 44 miligrams (a cup of tea brewed one minute may contain only 28); an ounce of chocolate, six miligrams; a cup of cocoa, five, and a 12-once cola drink, 35 to 49.

The FDA is expected to advise caution during pregnancy in using all these, as well as some pain-killers and over-the-counter wakeup pills that may contain as much as 100 milligrams apiece.