Although the tastes of mothers-to-be usually run along far more normal lines, the "pickles and ice cream" image is accurate in portraying the food cravings--and aversions--that sometimes accompany pregnancy. These tastebud changes often reflect changes in nutritional needs.

Such changes are partly due to the nourishment demands of the fetus and partly to other physiological variations that affect absorption and metabolism of nutrients. These changes help insure normal development of the baby and fill the subsequent demands of lactation, or nursing.

Exactly how nutrients are exchanged between mother and fetus is not understood. In the past it was viewed as a host-parasite relationship, with the fetus in the role of the parasite, taking whatever nourishment it required from the host mother. But recent research has shown that the fetus is not a perfect parasite. The fetus is sometimes more affected than the mother by lack of nourishment, and there is a relationship between maternal weight gain and growth and development of the fetus.

Pedro Rosso, M.D., of Columbia University's Institute of Human Nutrition, wrote in Nutritional Disorders of American Women that "contrary to the idea of fetal parasitism, there seem to be feedback mechanisms operating in the mother that would reduce the maternal supply line to the fetus when nutrients are in short supply."

Writing in Nutritional Impacts on Women, two English researchers, Frank E. Hytten, M.D., and Angus Thomson, said that changes in nutritional needs in pregnancy appear to be related to the body's adaptation to pregnancy because the changes occur too early to be responding solely to fetal needs. Such changes include a reduction of electrolytes, proteins, glucose, vitamin B-12, folate, vitamin B-6, and a rise in lipids, triglycerides, and cholesterol in blood.

The consequences of maternal malnourishment may include health problems for the mother and an infant of low birth weight who may have nutritional and other deficiencies.

Nutrients for the fetus come from the mother's diet, stored nutrients in the mother's bones and tissues, and synthesis of certain nutrients in the placenta. The placenta facilitates the transfer of nutrients, hormones, and other substances from mother to fetus.

According to a booklet by Rosly B. Alfin-Slater, Ph.D., titled Nutrition and Motherhood, if the mother is poorly nourished, the placenta does not perform its functions as well.

The Food and Nutrition Board of the National Academy of Sciences specifies certain increases in the Recommended Daily Dietary Allowances (RDAs) for pregnant and lactating women. More iron is needed not only because of fetal demands, but also because the mother's blood volume may be increased as much as 30 percent. Because the additional requirement for iron cannot be met by the usual American diet nor by existing stores in many women, iron supplements of 30 to 60 milligrams under supervision of a health-care professional are recommended.

The main effect of inadequate iron during pregnancy is iron deficiency anemia, which makes the mother less able to fight off an infection and less able to tolerate hemorrhaging during childbirth. It has been suggested that pica, the craving for substances with little or no nutritional value, may be associated with iron deficiency. Although pica occurs during pregnancy in a number of ethnic groups and geographic areas, in this country it is most prevalent among southern blacks. The most common substances eaten are dirt, clay, starch, and ice. The National Research council has noted that as many as 75 percent of the pregnant women attending southern health department clinics consumed starch and 50 percent ate clay. Concerns about the practice are several. First, eating these substances may take the place of eating nutritionally adequate food. Second, some pica substances, such as starch, are high in calories and may contribute to obesity. Third, some pica substances (such as charcoal, air fresheners, and mothballs) contain toxic substances. Fourth, the chemical makeup of some these substances (such as charcoal, air fresheners, and mothballs) contain toxic substances. Fourth, the chemical makeup of some of these substances interferes with the absorption of minerals. Although it is not known whether anemia is the cause or the effect of pica, the craving abates when the anemia is corrected.

To a certain extent, Mother Nature lends a hand in pregnancy by improving iron absorption. A woman who is not pregnant absorbs about 10 percent of the iron present in food consumed. A pregnant woman, however, can absorb up to twice as much. In addition, the fetus stores iron during the last month or two of gestation. Some good sources of iron are meat (especially liver and other organs), egg yolks, and legumes.

Pregnancy doubles a woman's need for folate (folic acid or folacin). However, there is not universal agreement on the necessity of folate supplements for all pregnant women. Women can get additional folate by eating more green leafy vegetables, certain fruits, and liver and other organ meats. Severe folate deficiency can result in a condition called megaloblastic anemia, which occurs most often in the last trimester of pregnancy. In this condition the mother's heart, liver and spleen may become enlarged, and the life of the fetus may be threatened. (Updated Information)

Because folic acid is crucial to cell multiplication, the fetus's needs are met before those of the mother. Therefore, the mother's health is more adversely affected at first. In contrast to the increased absorption of iron in pregnancy, folic acid absorption may be impaired by hormonal changes in pregnancy.

Pregnant women also have an increased need for vitamin B-6 and B-12. B-6 requirements usually can be met by eating more whole grains, milk, egg yolks, and organ meats. Vitamin B-12 is found in foods of animal origin, including eggs and milk products. Because B-12 occurs only in such foods, vegetarians who eat no eggs or cheese (vegans) should ask their health-care professionals about the necessity of B-12 supplements. (See "There's Something to Be Said for Never Saying 'Please Pass the Meat'" in the February 1981 FDA Consumer. Severe vitamin B-12 deficiency in pregnancy is rare.

A word about using vitamin and mineral supplements in pregnancy: If Taken, they would be at about RDA levels. Large doses of vitamins and minerals should be avoided. In animal studies, megadoses of vitamins A and D have resulted in fetal defects. The same is likely to be true in humans.

Pregnant adult women need an extra 400 milligrams of calcium daily. That's about 50 percent more than recommended for women 25 and older. Nearly all of the extra calcium goes into the baby's bones. this need can usually be met by consuming more dairy products. If there is not enough calcium in the mother's diet, the fetus may draw calcium from the mother's bones. Calcium deficiency in pregnancy may result in osteopenia (decreased bone density) in the mother.

Nature also helps supply the extra calcium needed in pregnancy by improving calcium absorption. Less is lost in urine and feces, and passage of calcium through the placenta to the fetus is facilitated.

A pregnant woman needs three or more servings of milk or other dairy products a day to get 1,200 milligrams of calcium. For women who are lactose intolerant, there area variety of low-lactose and reduced and reduced-lactose food products available. Sometimes calcium supplements are recommended by a woman's doctor. But pregnant women should not take calcium supplements such as bone meal and dolomite. FDA surveys have shown that some bone meal and dolomite products contain substantial amounts of lead. Lead can be harmful to both mother and fetus. Attitudes have changed about weight gain in pregnancy. In the past, pregnant women were told to limit gain to about 15 pounds. Higher weight gain was thought to be related to a number of problems. The most worrisome of these problems was toxemia (also called Pregnancy Induced Hypertension--PIH), a condition of unknown origin occurring after the 20th week of pregnancy and involving high blood pressure and protein in the urine or water retention of both. Although sudden large weight gain, water retention and blood pressure elevation continue to be recognized danger signs of toxemia, most physicians have come to agree that weight gain does not cause toxemia. The consequences of restricting weight gain, in fact, appear to be potentially more harmful, particularly to the fetus, than unrestricted weight gain, even in women who are overweight before becoming pregnant.

If a woman's calorie intake is restricted in pregnancy, she may not get enough protein, vitamins and minerals to adequately nourish her unborn child. Low-calorie intake can result in a breakdown of stored fat in the mother, leading to the production of substances called ketones in her blood and urine. The production of ketones is a sign of starvation of a starvation-like state. Chronic production of ketones can result in a mentally retarded child.

For these reasons, the National Academy of Sciences recommends that pregnant women eat an average of 150 calories more per day in the first trimester and 350 calories more per day in the two subsequent trimesters than they did before becoming pregnant. A total weight gain of about 25 to 30 pounds is usually recommended, with the actual pattern of gain considered more important than the number of pounds. Weight gain should be at its lowest during the first trimester, and should steadily increase, with the mother-to-be gaining the most weight in her third trimester, when the fetus and placenta are growing the most.

The effects of undernutrition on infant size is greatest when nutritional deprivation occurs during the final three months. Weight gain in the second trimester is due mostly to increases in tissue, blood volume, and fat stores, and enlargement of the uterus (womb) and breasts.

Arthur Alfin-Slater estimates that a 25-pound weight gain breaks down as follows: baby, 8 pounds; placenta, 1 pound; amniotic fluid, 1.5 pounds; breasts, 3 pounds; uterus, 2.5 pounds; and stored fat and protein, water retention, and blood volume, 8 pounds.

Along with increased total calories, pregnant women need high- quality protein daily, the approximate amount contained in two large eggs and 2 ounces of cheese or a 4-ounce serving of meat.

During pregnancy, fat deposits may increase by more than a third the total amount a woman had before she became pregnant. Most women lose this extra weight in the birth process or within several weeks thereafter. Breast-feeding helps to deplete the fat deposited during pregnancy. A woman who breast-feeds expends 600 to 800 more calories than one who doesn't. The woman who nurses he baby also has increased needs for specific nutrients (see chart).

The extra 600 to 800 calories a day includes both the nutritive value of the milk produced as well as the energy needed to synthesize the milk from lactose, protein and fat. Severely undernourished women produce less milk. However, obese women produce the same amount of milk as those of average weight. The amount of vitamins in human milk, particularly water-soluble vitamins such as C and the B complex, is closely related to that in the mother's diet. The concentrations of trace elements such as copper fluoride, and of fat-soluble vitamins, seem to be less dependent on the fluctuations in maternal eating habits.

Pregnancy is a natural, healthy state, and most changes in pregnant women occur without harmful effects. But some physiological changes have been topics of particular medical concern. In past years, the tendency of pregnant women to retain water has led to restriction of sodium intake. When water retention was severe, diuretics were frequently prescribed to avoid toxemia. However, views on sodium restriction have changed. today, there is considerable medical opinion that pregnancy is a "salt-wasting" condition--that is, one in which the body can use more salt than usual. Further, sodium deprivation may be harmful to the fetus. The sodium intake usually recommended in pregnancy is 2,000 to 8,000 milligrams a day, compared to the normally recommended 1,100 to 3,300 milligrams per day. However, pregnant women should be careful that their sodium intake does not greatly exceed this allowance.

Sugar is also an occasional concern in pregnancy. Virtually all women excrete more glucose (a form of sugar) in their urine when they are pregnant. This is one of the normal physiological adjustments pregnancy and is not a cause for concern in the majority of women. It is significant only in the few women who have a tendency towards diabetes and who may thus become diabetic during pregnancy.

Diabetic women should be closely monitored to make sure their blood sugar values are at or near normal. If maternal blood sugar rises too high, the increased sugar crossing the placenta can result in a large, overdeveloped fetus and an infant with blood sugar level abnormalities. Diabetic women may also suffer from a greater loss of some nutrients.

Nausea in early pregnancy is another condition that often can be managed nutritionally. Dr. Alfin-Slater's booklet suggests the following:

• Keep meals small, and avoid long period without food.

• Drink fluids between, but not with, meals.

• Avoid foods that are greasy, fried or highly spiced.

Improvements in the technological ability to diagnose birth defects early in pregnancy have focused attention on ways to correct certain fetal defects by manipulating the mother's diet. For example, researchers are investigating the use of vitamin- mineral supplements to prevent neural tube defects--that is, failure of the fetus's neural tube to close because of spinal cord abnormalities. Other investigators are researching ways maternal nutrition can help fetuses with inherited birth defects, usually inborn errors of metabolism, in which certain nutrients are not processed normally.

The effects of a woman's diet on her children start long before she becomes pregnant. Stores of fat, protein, and other nutrients built up over the years are called upon during pregnancy for fetal nourishment.