Listen to the old wives.

Sometimes the conventional wisdom of the ages is a lot more appropriate than the medical community cares to admit. For example, that long-scorned theory that a pregnant woman needs to eat more because she is "eating for two" is not so far off the mark.

Obstetricians and other prenatal health professionals now have found that over-emphasis on weight control during pregnancy has major inherent dangers, both to mother and unborn child. What's important is a nourishing, well-balanced diet, not the extra 20 or 30 pounds.

At the same time, eating for two has taken on new importance in terms of what the pregnant woman eats, and what she breathes as well.

The American Academy of Pediatrics has stated, flat out, that no drug has been proven safe for the unborn or nursing child. That is a good place to start. But there is plenty of evidence about specific harmful effects of some substances.

A startling study by the National Institute of Neurologic and Communicative Disease Disorders and Stroke, which followed 3,500 infants for up to 7 years, showed a strong relationship between pain-killing drugs administered during labor and neurologic problems in many of the infants during their first year of life, or even longer. Other studies have outlined the dangers of using diuretics or even salt-restricted diets to control water retention.

Then there are other dangers in common substances usually not regarded as drugs -- but which are.

Dr. Theodore M. King, director and professor of the Department of Gynecology and Obstetrics at the Johns Hopkins University Medical School, and two obstetrical nurses -- Irene Morelli, associate director of the Maryland Nurses Association, and Zoila Ortega Acevedo, a writer on health issues -- are conducting a series of seminars for childbirth nurses, midwives, technicians and educators.

They are especially concerned with spreading the word on such common drug-filled items as cigarettes, coffee, tea, colas, chocolate, herbal teas and soups, alcoholic beverages, as well as over-the-counter products (ranging from antihistamines in cold, sleep and anti-emetic preparations to aspirin and other analgesics, and even vitamin and mineral supplements).

"In the first place," says Dr. King, "there is no way the Food and Drug Administration can make it mandatory that pharmaceutical companies test new preparations in pregnant women . . . but even if you did drug safety studies you'd still have all the problems of the interactions of multiple agents in a single administered drug." This, Dr. King says, is the major problem with over-the-counter medicines which often contain combinations of asprin, alcohol, antihistamines or caffeine.

Caffeine: This ubiquitous central nervous system and heart stimulant recently has been linked to birth defects, and the FDA has issued an alert to pregnant women. Although no "safe" limits have been set, Acevedo told the workshop that fetal damage can be seen at around 700 milligrams of daily caffeine ingestion. "You can get there without even noticing," she said. A 5-ounce cup of coffee has 110 mg. Instant coffee has 66; tea, 45; cola, 50. Chocolate also contains caffeine, as do common drugs such as Anacin.

Herbal products: Full of potentially harmful chemicals, warn Acevedo and Morelli. "Who knows where they were harvested, or how they may have been contaminated." It is a mistake to assume that "natural" means "safe."

Alcohol: A central nervous system depressant. Excessive alcohol in pregnancies can cause congenital abnormalities and in extreme cases produce fetal alcohol syndrome, a life-threatening condition. The effects of alcohol are intensified if the woman smokes as well.

Cigarettes: The indictment against smoking in pregnancy continues to build. There are 4,000 substances in cigarettes and only a few have been tested. A few are enough: nicotine, a CNS stimulant; carbon Monoxide, interferes with the absorption of oxygen; cyanide, an out-and-out poison.

Smoking in pregnancy can cause complications for the mother, retard growth in the infant and has been linked to hyperactivity, respiratory problems, even retardation and other symptoms of brain damage. Even exposure to the smoke from the cigarettes, cigars and pipes of others can be damaging. The fact is simple: Cigarette smoke blocks oxygen to the developing fetus.The potential for damage is catastrophic. A new book, "Smoking for Two; Cigarettes and Pregnancy" by Peter A. Fried and Harry Oxorn ($10, Macmillan) should be required reading for any woman who smokes and is, or plans to be, pregnant.

A footnote: Dr. King, Irene Morelli and Zoila Acevedo are dedicated and knowledgable. The seminars, however, are sponsored by McNeil Consumer Products Co., which makes, among other things, the acetaminophen analgesic, Tylenol. Literature which accompanied the seminar implies that because studies have indicated some problems associated with aspirin, especially in the last weeks of pregnancy, "Tylenol" might be better. In fact, no studies document its safety in pregnancy, and its use in pregnancy should be encouraged no more than the use of any other drug.