Nutrition in Pregnancy1
R. Elaine Turner
1Abbreviations: AI, adequate intake; BEE, basal energy expenditure; BMI, body mass index; DHA, docosahexaenoic acid; DRI, dietary reference intake; EAR, estimated average requirement; FAS, fetal alcohol syndrome; GDM, gestational diabetes mellitus; GWG, gestational weight gain; IOM, Institute of Medicine; IUGR, intrauterine growth restriction; LBW, low birth weight; LGA, large for gestational age; NTD, neural tube defect; PKU, phenylketonuria; RDA, recommended dietary allowance; TEE, total energy expenditure; UL, tolerable upper intake level.
Optimal nutrition is integral to a healthy pregnancy, which can be described as “without physical or psychological pathology in mother or fetus, results in delivery of a healthy baby” (1). Although the influence of poor nutritional status on adverse pregnancy outcome was documented early in the twentieth century, retrospective studies considering the effects of food shortages in the Netherlands during World War II clearly identified the influence of diet on pregnancy outcome (2, 3, 4). Nutrition can affect the mother’s health and risk of pregnancy complications; it also affects the growth and development of the fetus, risk of birth defects, and health of the infant at delivery. Further studies have linked both undernutrition and overnutrition during pregnancy to increased risks of obesity, coronary heart disease, hypertension, diabetes, metabolic syndrome, and psychiatric disorders in the children; these findings suggest a persistent change in gene expression in response to the intrauterine environment (5, 6, 7, 8).
CURRENT PUBLIC HEALTH OBJECTIVES RELATED TO PREGNANCY AND NEONATAL HEALTH
Maternal and infant health are important predictors of the future health of the nation’s citizens. As identified in Healthy People 2020, a major public health goal is to “improve the health and well-being of women, infants, children, and families” (9). Public health issues related to maternal and child health include morbidity and mortality of pregnant and postpartum women; fetal, perinatal, and infant mortality; birth outcomes; prevention of birth defects; and access to preventive care. Progress has been made toward objectives related to fetal, infant, and maternal deaths; prenatal care; and prevention of neural tube defects (NTDs); however, the percentages of low birth weight (LBW) and preterm delivery have increased (10). Healthy People 2020 objectives continue to emphasize the importance of nutrition, prenatal care, and preconception health in improving the health of mothers and infants (8).
PRECONCEPTION HEALTH
Nutritional status before pregnancy is a key factor in overall maternal health and in the risk of birth defects. Women who are contemplating pregnancy can make dietary and lifestyle changes that will reduce the risk of poor pregnancy outcome. The Centers for Disease Control and Prevention identified preconception risk factors for poor pregnancy outcomes (Table 52.1) and developed 10 recommendations to improve preconception health (11). Folic acid supplements before and during the early stages of pregnancy reduce the risk of NTDs and other birth defects. Ideally, all women of childbearing age should be consuming 400 µg/day of folic acid in addition to folate provide through foods (12) because nearly one half of all pregnancies in the United States are unplanned (13). Women following vegan or other strict vegetarian diets should also take supplemental vitamin B12 because the status of this vitamin is another risk factor for NTDs (12).
Preconception iron status is important to reducing the risk during pregnancy of iron deficiency and anemia, which, in turn, can lead to intrauterine growth restriction (IUGR) and preterm birth (14). Preconception care should include screening for iron deficiency anemia. Multivitamin and mineral supplementation may help to improve nutritional status in women who are following inappropriate diets, are avoiding numerous foods or groups of foods, are underweight, are trying to lose weight, or are abusing alcohol.
Achieving a healthy weight before pregnancy can improve chances of conception and pregnancy outcome and may improve lactation (13, 15). Women who are obese at the start of pregnancy are at greater risk of gestational diabetes mellitus (GDM) and preeclampsia and of experiencing induced labor and cesarean section. Obese women may also have more difficulty initiating breast-feeding (15, 16, 17). Infants born to women who were obese before pregnancy are at increased risk of congenital abnormalities, NTDs, stillbirth, macrosomia, and obesity later in life (15). Physical activity can help to improve weight and nutritional status; however, the amount of physical activity needed daily for weight management, chronic disease risk reduction, and enhanced physical fitness varies (18, 19, 20).
TABLE 52.1 PRECONCEPTION RISK FACTORS FOR POOR PREGNANCY OUTCOME | ||||||||||||||||
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Management of preexisting chronic disease is another important element of preconception planning. Women with hypertension are at risk of maternal, fetal, and neonatal morbidity and mortality. The severity of hypertension and the presence of preeclampsia affect pregnancy outcomes (21).
Diabetes increases the risk of birth defects, especially defects of the heart and central nervous system, and it also increases the risk of spontaneous abortion (21). Attaining good blood glucose control before conception and during organogenesis can substantially reduce risks.
Approximately 3000 to 4000 women of childbearing age in the United States have phenylketonuria (PKU) without severe mental retardation (22). To prevent mental retardation, microcephaly, and congenital heart disease in the infant, pregnant women with PKU must resume a low-protein, amino acid-modified diet during pregnancy (22). Ideally, women with PKU should resume the diet before conception to regain control of blood phenylalanine and then maintain continued tight control throughout pregnancy.
MATERNAL PHYSIOLOGIC CHANGES DURING PREGNANCY
Numerous anatomic, biochemical, and physiologic changes occur during pregnancy to maintain a healthy environment for the growing fetus without compromising the mother’s health. Many of these changes begin in the early weeks of pregnancy, and together they regulate maternal metabolism, promote fetal growth, and prepare the mother for labor, birth, and lactation. A review of the physiologic changes during pregnancy sets the stage for understanding the changes in nutrient requirements that accompany pregnancy.
Maternal plasma volume begins to expand near the end of the first trimester, with a total increase in volume of 50% by 30 to 34 weeks of gestation. Red blood cell production is stimulated with a total increase in red blood cell mass of approximately 33%. Hematocrit levels decline until the end of the second trimester, by which time red blood cell synthesis is synchronized with plasma volume increase. Declining concentrations of plasma proteins and other nutrients are expected because of the expansion of blood volume. Poor plasma volume expansion predicts a poorly growing fetus and poor pregnancy outcome (23).
Cardiac output increases approximately 30% to 50% during pregnancy. Elevated cardiac output occurs in response to increased tissue demands for oxygen and
is accompanied by an increase in stroke volume. The size of the heart increases by approximately 12%, probably because of the increased blood volume and cardiac output. Systemic blood pressure declines slightly during pregnancy, with the majority of the change occurring in diastolic pressure (5 to 10 mm Hg). Diastolic pressure returns to prepregnancy levels near term.
is accompanied by an increase in stroke volume. The size of the heart increases by approximately 12%, probably because of the increased blood volume and cardiac output. Systemic blood pressure declines slightly during pregnancy, with the majority of the change occurring in diastolic pressure (5 to 10 mm Hg). Diastolic pressure returns to prepregnancy levels near term.
Respiratory changes support increased maternal and fetal requirements for oxygen. As the uterus enlarges, the diaphragm is elevated, which reduces lung capacity by about 5%, and residual volume is reduced by approximately 20%. Tidal volume increases as pregnancy progresses, resulting in increased alveolar ventilation and more efficient gas exchange, given that oxygen consumption increases only 15% to 20%. Respiration rate increases only slightly.
The kidneys increase slightly in both length and weight during pregnancy; and the ureters elongate, widen, and become more curved. Glomerular filtration rate increases by approximately 50%, and renal plasma flow rate increases by 25% to 50%. Renin levels increase early in the first trimester and continue to rise until term. Most pregnant women are resistant to the pressor effects of the resulting elevation of angiotensin II levels, but enhanced renin secretion may help to explain preeclampsia. A marked increase in excretion of glucose, amino acids, and water-soluble vitamins occurs, probably because the higher glomerular filtration rate presents higher levels of nutrients than the tubules can reabsorb.
Changes along the gastrointestinal tract support the increased demand for nutrients during pregnancy. Appetite increases, although initially this may be offset by nausea and vomiting. Motility of the gastrointestinal tract is reduced by increased levels of progesterone that, in turn, decrease the production of motilin, a hormone that stimulates smooth muscle in the gastrointestinal tract. Elongation in gastrointestinal transit time occurs largely in the third trimester of pregnancy and is not accompanied by a change in gastric emptying time (24). Gallbladder emptying time is reduced and often incomplete.
The basal metabolic rate rises by the fourth month of gestation and is usually increased by 15% to 20% by term. An elevated basal metabolic rate reflects the increased demand for and consumption of oxygen. Most (50% to 70%) of the energy needs of the fetus are provided by glucose, with approximately 20% coming from amino acids and the remainder derived from fat. Use of fatty acids for fuel is enhanced in the mother to conserve glucose for use by the fetus.
TABLE 52.2 GESTATIONAL WEIGHT GAIN RECOMMENDATIONS | ||||||||||||||||||||||||||||
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WEIGHT GAIN
Optimal birth weight is influenced by maternal weight gain. In 2009, the Institute of Medicine (IOM) released updated recommendations for gestational weight gain (GWG) (23). These recommendations, based on prepregnancy body mass index (BMI), reflect the GWG and rate of weight gain associated with best pregnancy outcome (Table 52.2).
Determinants of Gestational Weight Gain
Numerous factors potentially influence the amount of weight gained during pregnancy, including environmental factors, maternal genetics and body size, medical and psychologic conditions, and behavioral factors. Limited evidence is available to ascertain the strength of most of these relationships (23). Prepregnancy BMI is likely the best independent predictor of GWG (23). A review of several studies showed that mean weight gain by underweight (BMI <18.5 kg/m2) and normal weight (BMI 18.5 to 24.9 kg/m2) women was within the new IOM recommendations, whereas mean GWG of overweight (25.0 to 29.9 kg/m2) and obese (≥30 kg/m2) women was higher than the new recommendations (23).
Effect on Fetal and Maternal Outcomes
Studies show a linear relationship between GWG and birth weight for gestational age. Poor weight gain is associated with poor fetal growth, LBW, small for gestational age birth weight, and risk of preterm delivery (23, 25). Carmichael and Abrams (25) found that a marked acceleration or deceleration of gain toward the end of pregnancy was associated with lower gestational age and risk of spontaneous preterm delivery. Low GWG is also associated with failure to initiate breast-feeding (23).
Excessive weight gain affects infant growth, increases the chance of large for gestational age (LGA) birth weight and cesarean delivery, and is associated with higher body fatness in childhood. Women who are overweight or obese are more likely than women of normal weight to gain more weight than is recommended (26), and exceeding recommendations was found to be more likely in low-income women (27). Excessive GWG is further associated with postpartum weight retention and future overweight or obesity (23).
Ideally, weight gain recommendations should be individualized to promote best outcomes while reducing risk for excessive postpartum weight retention and reducing the risk of later chronic disease in the child. The most effective approach for reducing negative outcomes associated with GWG is for women to be within the normal range for BMI at the time of conception (23). Even small weight gains between pregnancies increase the risk of maternal complications and stillbirth (28). Further, advice about appropriate GWG should be given during prenatal care, with tracking of actual weight gain and referral to services for counseling on diet and physical activity as needed (23).
ENERGY AND NUTRIENT NEEDS
To support the growth of the fetus and the health of the mother, requirements for energy and for most vitamins and minerals are higher during pregnancy.
Energy
Energy is needed to support basal energy expenditure (BEE), physical activity, the thermic effect of food, and, in pregnant women, growth of the fetus and deposition of maternal tissues. BEE increases because of the enhanced metabolism of the uterus and fetus and the increased work of the heart and lungs. Increased BEE represents the major component of increased energy requirements. Studies estimate the cumulative increase in BEE at 106 to 180 kcal/day, although variation among subjects is substantial (26). Late in pregnancy, the fetus uses approximately 56 kcal/kg/day, which represents about one half of the increment of BEE.
The theoretic energy cost of energy deposition can be estimated from the amount of protein and fat deposited (19). Mean total energy deposition is 39,862 kcal (180 kcal/ day). Analysis of studies employing the doubly labeled water method shows a median change in total energy expenditure (TEE) of 8 kcal/gestational week. The estimated energy requirement for pregnancy is thus derived from the sum of TEE for a nonpregnant woman plus 8 kcal/gestational week plus 180 kcal/day for energy deposition. This increase in recommended energy intake is suggested only for the second and third trimesters because TEE changes little in the first trimester, and weight gain is minimal. Therefore, during the second trimester, an additional 340 kcal/day greater than nonpregnant energy requirements is recommended. This increase climbs to 452 kcal/day extra in the third trimester.
Ultimately, the best method for assessing the adequacy of energy intake is to monitor GWG. The recommended balance of energy sources during pregnancy is the same as for nonpregnant women: 10% to 35% as protein, 45% to 65% as carbohydrate, and 20% to 35% of kcal as fat (19). IOM recommendations for nutrient intake during pregnancy are summarized in Table 52.3.
Protein
During pregnancy, whole body protein turnover increases, and substantial amounts of protein are accumulated by the growth of the fetus, uterus, blood volume, placenta, amniotic fluid, and maternal skeletal muscle (19). Considering protein deposition over the last two trimesters, the recommended dietary allowance (RDA) increases by 25 g/ day. For a reference woman weighing 57 kg, this is an additional 0.27 g/kg/day for a total of 1.1 g/kg/day during pregnancy.
Carbohydrate
The fetus uses glucose as its major energy source. The transfer of glucose from mother to fetus is estimated at 17 to 26 g/day. By the end of pregnancy, all this glucose is thought to be used by the fetal brain (19). The estimated average requirement (EAR) for carbohydrate increases from 100 g/day to 135 g/day, which translates to an RDA for carbohydrate for pregnant women of 175 g/day.
Fat
Fat is a major source of energy for the body and aids in the absorption of fat-soluble vitamins and carotenoids. Some studies have shown lower maternal concentrations of arachidonic acid in plasma and red blood cell phospholipids (19). However, no evidence indicates that supplementation with n-6 fatty acids has any beneficial effect on fetal growth and development. The developing brain accumulates large amounts of docosahexaenoic acid (DHA) during prenatal and postnatal development, continuing through the first 2 years of life. Fetal tissue has active desaturases to allow DHA formation from α-linolenic acid. No evidence has been found of physiologic benefit to the infant of increasing DHA intake during pregnancy if the diet meets n-3 and n-6 requirements. Therefore, adequate intake (AI) values for essential fatty acids during pregnancy are based on median intakes among pregnant women in the United States: 13 g/day for linoleic acid and 1.4 g/day for α-linolenic acid.
Fat-Soluble Vitamins
Vitamin A is important for regulation of gene expression and for cell differentiation and proliferation, particularly for the development of the vertebrae and spinal cord, the
limbs, heart, eyes, and ears. Direct studies of vitamin A status are lacking, but the increase in maternal requirement of 70 µg/day as retinol activity equivalents is estimated based on the amount of vitamin A assumed to be accumulated by the fetal liver (29).
limbs, heart, eyes, and ears. Direct studies of vitamin A status are lacking, but the increase in maternal requirement of 70 µg/day as retinol activity equivalents is estimated based on the amount of vitamin A assumed to be accumulated by the fetal liver (29).
TABLE 52.3 RECOMMENDED DIETARY ALLOWANCE, ADEQUATE INTAKE, OR ACCEPTABLE MICRONUTRIENT DISTRIBUTION RANGE AND TOLERABLE UPPER INTAKE LEVEL FOR NUTRIENTS DURING PREGNANCY | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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