It is becoming increasingly evident that the outcome of pregnancy for both mother and child is affected in important ways by nutritional status.1 Many of the dietary, metabolic, psychosocial, and cultural factors that interact to determine nutritional status have special characteristics during adolescence. These factors and their variance during adolescence from what can be expected both earlier and later in life are the subject of this article. We hope that the pediatrician will be able to help the pregnant teenager achieve a more favorable outcome of her pregnancy than might otherwise be the case by using the concepts outlined below.
A clinic expressly for prenatal care of pregnant adolescents aged 15 and under has been in operation for five years at the University of Maryland Hospital in Baltimore, operating under the Division of Adolescent Medicine. Primary physicians in the clinic are pediatricians with training in adolescent medicine. Nutritionists, nurses, social workers, psychologists, and obstetric consultants have all been integrated into the clinical care of these teenage girls.
The goal of the clinic has been to reduce the disproportionate number of low-birth-weight infants delivered in this high-risk population. Nutritional support has been a key factor in the success of this program.2 The purpose of nutritional support is twofold: to provide for optimal fetal growth and to maintain or improve the status of the mother's nutrition during and after gestation.
Nutritional assessment is an integral part of the patient's initial work-up, and nutrition counseling is a part of each subsequent visit. The review of each patient's case by all professionals concerned with her care ensures consistency in the recommendations we make to each of these teenage mothers.
In this article we will provide an overview of energy requirements of the pregnant teenager; discuss the need for protein, iron, calcium, sodium, and vitamins in her diet; and suggest steps that can be taken when the adolescent patient is not likely to receive an adequate diet.
ENERGY REQUIREMENTS IN PREGNANCY
Monitoring of energy balance is the cornerstone of nutritional support of pregnancy in adolescence. Energy is necessary to maintain cellular function and to promote growth of maternal and fetal tissue. Studies have shown that in moderately malnourished populations, caloric supplementation during gestation will promote increase in birth weight, which is especially important in that low birth weight is a major factor in perinatal morbidity and mortality.3
Energy is derived from the dietary components of carbohydrate, fat, and protein. In the past, various therapeutic regimens have attempted to restrict one or another of these components in order to prevent toxemia or large babies, or both. Scientific opinion now favors a balance among the various components; therefore, nutritional assessment and counseling should take this into account. Carbohydrate intake of 45 to 50 percent of calories is sufficient to prevent ketosis and promote optimum utilization of protein while assuring a palatable diet that can meet vitamin and mineral needs. About 20 percent of dietary calories should be derived from protein and 30 to 35 percent from fat. Table 1 presents energy as well as other nutrient needs of adolescents, both pregnant and nonpregnant.
NUTRIENT NEEDS OF ADOLESCENTS
The B vitamins (thiamine, riboflavin, and niacin) are important in the intracellular utilization of energy. Thiamine is needed to conven the end product of glucose metabolism, pyruvate, to acetylcoenzyme A. Niacin and riboflavin are key coenzymes in the mitochondrial respiratory processes. Dietary sources of these vitamins are widespread. Whole grains, legumes, organ meats, and pork are high in thiamine, while milk, cheese, lean meats, and leafy green vegetables contain riboflavin. Niacin can be found in all the above or can be synthesized from dietary tryptophan.
Adequacy of energy intake is best monitored by estimates of intake and close monitoring of weight gain and fetal growth. A number of factors act synergistically to decrease available energy during pregnancy in adolescence; these factors may account for the disproportionate number of towbirth-weight infants delivered, especially by those 15 years and under.
First, adolescents carrying their pregnancy to term are often from economically disadvantaged populations who are more likely to be at nutritional risk before conception . It is well documented that prepregnancy weight and height have a strong influence on birth weight and prematurity. Second, the younger adolescent who has not completed her own growth is likely to have an energy requirement higher than that of other pregnant women. Peak height spurt generally occurs before menarche, but skeletal growth may continue to age 17; therefore, an increment in recommended caloric intake is generally added to cover growth.4
Third, there are a variety of sociocultural factors that may interact to restrict caloric intake for the pregnant adolescent. The positive value our society gives to slimness strongly affects teenage girls. Caloric deficits of 300-400 calories/day are commonly found in nutritional surveys. This tendency can be exacerbated when perceived social pressures from family or peers cause the young woman to hide her pregnancy by restricting her weight gain.
Experts suggest that it is the quality and pattern, as well as the amount of weight gain, that are important in securing optimum pregnancy outcome.6 Figure 1 shows the mean weight gain in pounds by gestational age in weeks. In the first trimester, there is generally only minimal gain, while during the second and third trimesters, 1 pound per week is expected. The second trimester's gain is primarily maternal, while the third's is primarily fetal.
Of the 27 pounds gained during the average pregnancy, approximately 4 pounds is accounted for by the increase in maternal blood volume, 4 by the increase in uterus and breast tissue, and 4 by extracellular fluid. Two pounds represents maternal adipose tissue. The placenta accounts for about 1.5 pounds, and the infant at term is 7 pounds. Amniotic fluid accounts for the remaining 4-plus pounds.6 As Figure 1 indicates, most pregnant women gain an average of 1 pound per week during the last 27 weeks of pregnancy.
It is not clear whether a chart for pregnant adolescents should show an additional gain for maternal growth. The issues relevant in studying this problem are complex.
At the present, most authorities believe any plan to impose restrictions on the pregnant woman's weight gain should be approached with caution. The problems of pre-eclampsia and eclampsia will be discussed below.
At the University of Maryland Adolescent Clinic, weight gain and fetal growth are monitored every two to three weeks until 36 weeks* gestation, when weekly visits are the rule. If there is a suggestion of poor maternal weight gain or intrauterine growth retardation, medical, obstetric, nutritional, and psychosocial assessment is utilized to define what factors are likely to be responsible. Intervention is begun early, and hospitalization is sometimes used in a manner similar to its use in the work-up of failure to thrive in general pediatrie practice. Treatment of medical complications, nutritional assessment, psychologic evaluation, and in-hospital observation have sometimes reversed failure to gain or weight loss in our adolescent patients.
Figure 1. Prenatal weight -gain grid. Pattern of normal prenatal weight gain. Source: Food and Nutrition Board, National Research Council.'
The nutrient requirements that have special significance to the pregnant teenager include proteins, minerals, and vitamins.
Protein is especially important in the diet of pregnant adolescents because cellular growth, function, and structure require the turnover and metabolism of the component amino acids. Studies aimed at defining protein requirements use nitrogen(lgm. = 6. 25 gm. of protein) as the measured element.
The problem of determining optimum protein intake for pregnant adolescents is more complicated than that of determining their energy needs. One of the reasons for this is that tissue growth does not occur in a linear fashion during pregnancy (Figure 1), and therefore the adolescent girl's protein needs will vary with the stage of gestation. Another factor is the ability to digest, absorb, and utilize protein from the diet; this varies according to the animal or vegetable origin of the food and its component amino acids.
Most important of all, protein utilization depends on adequacy of caloric intake. When a person's caloric intake is insufficient for her needs, protein will be used for energy rather than for structural requirements. Calculating nitrogen needs from the amount needed by a nonpregnant teenage girl and adding the amount contained in maternal and fetal tissues yields a figure for daily protein intake only half as large as that found when nitrogen -bal an ce studies are done.8 It is important, therefore, to use recommended protein dietary allowances as guidelines but not as indications of what the upper limits of protein intake should be.
Dietary studies have shown U.S. teenagers in general to be protein consumers. Fortunately, foods high in protein content are also high in minerals and B vitamins; therefore, protein intake should not be restricted. Folie acid and vitamin Bj2 are linked closely with the metabolic function of protein in new-tissue synthesis. Besides basic requirements for energy and structural amino acids, the cell needs deoxyribonucleic acid (DNA) and ribonucleic acid (RNA),synthesis for division and growth. Folie acid is a necessary cellular component in both DNA and RNA precursor synthesis.
Folate deficiency in terms of serum level has been found in as many as 60 percent of patients in some series, but the significance of this finding is moot. Animal studies of folate deficiency have shown fetal malformation, abortion, and abruptio placentae, and correlations have also been shown in human beings by some investigators.4 Other studies have failed to replicate these findings.4 At this point, the best available data suggest an oral supplement of 200-400 mg12-/day of folie acid, at least during the latter half of pregnancy. Dietary lack of B12 is likely to be seen in a pregnant teenager only if she is a strict vegetarian or has been on antibiotic therapy long enough to destroy gastrointestinal flora.
Mineral requirements of the pregnant adolescent are shown in Table 1 , Those of special relevance to adolescent mothers are iron, calcium, and sodium.
Iron is needed - in conjunction with adequate calories, protein, and folate - for the formation of maternal and fetal red blood cells. Fetal blood formation occurs in the third trimester; maternal expansion occurs primarily in the second trimester.
The fetus requires 250 mg. of elemental iron, and this is obtained at maternal expense, even in the face of maternal iron deficiency. The placenta at term contains 150 mg, of iron. Maternal erythrocyte mass increases 20 to 30 percent, which requires 500 mg. of elemental iron. The total iron need for an entire pregnancy is thus 900 mg., or 6 mg./day over the second half of pregnancy, when intervention is usually made. The average diet contains 10-15 mg. /day and, given 10 percent absorption, thus provides 1-2 mg. /day. Storage in reticuloendothelial cells may provide up to 300 mg., but one-third of U.S. women have no storage iron.6 Of teenagers in the Ten State Nutrition Survey, many were found to be at risk for iron deficiency.4 Therefore, it is recommended that iron supplementation be routine in pregnancy. At the University of Maryland Hospital, 325 mg. of ferrous sulfate daily (60 mg. of elemental iron) is prescribed throughout pregnancy for those with normal iron stores as assessed by serum ferritin. For those found to be iron-deficient (i.e., ferritin less than 10 mg./100 ml.), 325 mg. of ferrous sulfate is given three times a day.
Calcium requirements increase during the pubertal growth spurt. As Table 1 indicates, the recommended dietary allowance for the nonpregnant adolescent is 1,200 mg./day. The pregnant adolescent needs an intake one-third larger, or 1,600 mg. One reason for this is that growth and calcification of the fetal skeleton and dentition require an additional 30 gm. of calcium deposition during pregnancy. This is less than 5 percent of maternal stores, so the fetal effects on the mother's stores are not seen even when her calcium intakes are quite low. We do not know what effect low calcium consumption during pregnancy will have over the long term, particularly in relation to the mother's skeletal disorders in later life. That is why the recommended intake of calcium for the pregnant adolescent has been set at 1,600 mg./day.
Milk is an excellent source of dietary calcium. It contains 1.2 gm. of dietary calcium per quart, as well as 400 international units (I. U.) of vitamin D and 35 gm. of protein. Milk, consequently, has been considered by many to be a perfect food for the pregnant teenager. Those who do not tolerate it well (whether on the basis of Jactase deficiency or as a matter of personal preference) may need calcium supplements.
Sodium balance in pregnancy has been discussed by Pitkin6 in the light of some new findings that necessitate a change in the concept of pregnancy as a salt-retaining state. Glomerular filtration in pregnancy is increased by as much as 50 percent, resulting in an increased filtered sodium load. This is compensated for by elevation in aldosterone production, which promotes tubular résorption of sodium.
Pregnancy, therefore, has more features of salt depletion than of salt overload. Consequently, it is unphysiologic to restrict salt intake and prescribe diuretics for the pregnant woman to prevent preeclampsia. It is now clear, furthermore, that preeclampsia is a volume-contracted state and that the thiazides heretofore used to treat it may compromise placental function. In the University of Maryland Hospital Adolescent Clinic, salt restriction is limited to a no-added-salt policy for patients with pre-eclampsia; thiazide diuretics are prescribed Only when necessary in patients with chronic hypertension or cardiac disease.
Vitamin requirements are shown in Table 1; the needs of the pregnant teenager for B-group vitamins and folacin were discussed above.
With the exception of iron and folacin, dietary supplements are unnecessary for the pregnant adolescent who is receiving an adequate caloric intake from a variety of food sources. For those who do not have adequate diet, counseling sessions with a nutritionist skilled in working with adolescents is invaluable. When the counseling fails, supplemental energy, protein, minerals, and vitamins may be necessary to bring the intake to a level approximating that in Table 1. It is always necessary to assay other factors, including growth, weight gain, physical exam, and laboratory values, in addition to intake data in order to rationally plot a nutritional strategy.
The authors would like to acknowledge their appreciation to Cheryl McKeen and Susan McFaul for preparation of the manuscript.
1. Jacobson, H. N. Diet in pregnancy. N. Eng/. /. Med. 297 (1977), 1051-1053.
2. Felice, M., Granados, J., Anees, I., and Heald, F. P. A different approach to teenage pregnancy. Paper presented at the International Society for Adolescent Medicine, May 8, 1979.
3. Lech t ig, ?., et al. Effect of food supplementation during pregnancy on birth weight. Pediatrics 56 (1975), 508-520.
4. Worthington, B. S., Vermeersch. I-, and Williams, S. R. Nutrition in Pregnancy and Lactation. St. Louis: C. V. Mosby Company, 1977.
5. Food and Nutrition Board, National Research Council, National Academy of Sciences. Recommended Dietary Allowances, Eighth Edition. Washington, D-C. : U.S. Government Printing Office, 1974.
6. Pitkin, R. M. Nutritional influences during pregnancy. Med. Clin. North Am. 61 (1977), 3-16.
7. Committee on Maternal Nutrition, Food and Nutrition Board, National Research Council, National Academy of Sciences. Maternal Nutrition and the Course of Pregnancy. Washington, D. C.: U.S. Government Printing Office, 1970.
8. King, J. C, Galloway, O. H., and Margen, S. Nitrogen retention, total body "K and weight gain in teenage pregnant girls. J. Nutr, 1 03 (1973), 772.
NUTRIENT NEEDS OF ADOLESCENTS