Maternal and fetal normal and abnormal nutrition

Sarah Louise Killeen, Eilleen C. O’Brien, and Fionnuala M. McAuliffe

Introduction

Good maternal nutrition throughout pregnancy provides a supportive environment for fetal growth and development. Maternal dietary intake is a modifiable risk factor for pregnancy-related complications and is therefore an important consideration in the management of pregnant women. In addition to pregnancy and infant outcomes, maternal nutrition can influence the health and development of offspring from childhood into adulthood, as well as the future health of the mother (1–5). Good maternal nutrition can be defined as a diet that meets macronutrient and micronutrient requirements in pregnancy through the consumption of high-quality/bioavailable foods in the right proportions.

Dietary requirements

Macronutrients

Macronutrient requirements increase in pregnancy to support fetal development and tissue deposition. In the first trimester, this increase is only marginal, and during this time, pregnant women should focus on following general healthy eating guidelines (6). Sufficient, but not excessive, energy intake is essential for a healthy pregnancy and appropriate gestational weight gain (GWG). Basal metabolic rate has been shown to increase by 5%, 11%, and 25% in trimesters 1, 2, and 3, respectively (7). This increases energy requirements by approximately 85, 285, and 475 kcal/day during the first, second, and third trimesters, respectively; however, this will depend on individual factors, including pre-pregnancy weight, age, and physical activity (8,9). Women carrying twins or adolescent mothers may have even greater energy requirements (10,11).

The ratio of macronutrients contributing to total energy intake should not change in pregnancy, except in the case of an unbalanced pre-pregnancy diet. There is a moderate increase in carbohydrate requirements in pregnancy of approximately 45 g per day (6). Protein requirements are increased by approximately 1, 9, and 31 g per day for the first, second, and third trimesters (8). While the recommendations for dietary fat are unchanged in pregnancy compared to healthy eating guidelines, the importance of polyunsaturated fatty acids (PUFAs) should be highlighted as PUFA status declines during pregnancy and is essential for fetal neurological and eye development (6). Adequate intake can be achieved through the consumption of oily fish once to twice a week. More frequent consumption should be avoided to limit methyl mercury and polychlorinated biphenyls that may harm fetal development. Despite this, women should be discouraged from avoiding fish altogether as non–fish eaters may have increased risk of low birth weight (LBW) infants, and intake of fish once to twice a week has been shown to be safe (6,13). In fact, omega-3 PUFA supplementation has been shown to reduce the incidence of preterm birth, although it may increase the incidence of postterm pregnancies (14).

An unbalanced diet can be characterized by inadequate or excessive intakes of different nutrients that may have negative impacts on health. This may be of concern among women with obesity, who may be paradoxically malnourished in the context of overnutrition due to the consumption of energy-dense but nutrient-poor diets (15). Obesity has also been shown to be negatively associated with maternal iron, vitamin B12, and folate status (16,17). In addition, obesity negatively impacts on circulating vitamin D levels as vitamin D is a fat-soluble vitamin and is sequestered by adipose tissue. This means that pregnant women with obesity may have lower vitamin D status despite higher dietary intakes.

Micronutrients

While there is some physiological adaption that occurs in pregnancy to maintain nutritional status (e.g., increased intestinal absorption of nutrients), inadequate dietary intake during pregnancy may lead to inadequate nutrient provision to the growing fetus as micronutrients are transferred across the placenta. Alternatively, maternal nutrient stores may be depleted to ensure adequacy for the growing fetus, which negatively impacts on maternal health. Certain nutrients are at greater risk of this due to variance in transfer mechanisms. Micronutrients of concern in pregnancy include iron, folic acid, iodine, vitamin B12, vitamin D, calcium, and zinc (6). It is thus not surprising that multiple micronutrient supplements, which provide a variety of key nutrients at the Recommended Dietary Allowance (RDA) level, have been shown to be superior to single-nutrient supplements in the prevention of pregnancy-related complications (18). Table 2.1 gives details of recommended nutrient intakes for pregnant women (19,20).

Iron requirements increase during pregnancy by up to 50%, and despite physiological adaptations to maintain iron status, iron deficiency is common, with 38% of pregnant women affected worldwide (6,21). Cessation of menstruation helps maintain iron stores during the first trimester of pregnancy; however, blood volume and red blood cell mass increase from mid-pregnancy to provide sufficient oxygen to support fetal growth, and this increases iron requirements (21). Insufficient intake can lead to maternal anemia that in severe cases can increase the risk of mortality during childbirth (22). Women who have low habitual intake of bioavailable heme dietary sources (red meat, oily fish, and dark poultry meat) may benefit from supplementation with low-dose iron (e.g., 16–20 mg/day); in the case of maternal anemia, high-dose supplementation will be essential to replete iron stores (e.g., 100–200 mg elemental iron per day) (23).

Calcium is integral to bone and teeth and account for 99% of total calcium stores. Approximately 30 g of calcium is accreted by the fetus during pregnancy, mostly during the third trimester. Due to physiological adaptations in intestinal absorption, calcium requirements are not increased in pregnancy; however, in the event of inadequate dietary intake, maternal bone mineral will be resorbed so that fetal calcium requirements are met (24). Therefore, adequate daily intake should be encouraged in all pregnant women, which can be achieved through dairy products or fortified alternatives. Calcium intake is also inversely related to blood pressure in pregnancy as it is involved in vasoconstriction and dilation. Therefore, calcium supplementation in pregnant women with habitually low calcium intakes may reduce the risk of hypertensive disorders of pregnancy and associated conditions. This, however, may have negative impacts on maternal bone mineral content (6,25). In addition, vitamin D is essential for calcium absorption (26). Maternal vitamin D deficiency may lead to abnormal bone health in the newborn, LBW, hypocalcemia, and potentially, cardiac failure (6,27). Dietary sources of vitamin D are limited to oily fish, meat, eggs, dairy products, and fortified foods. Inadequate dietary intake is common in pregnancy (28). This means that routine supplementation to avoid deficiency is advisable, especially for women who are not exposed to sufficient ultraviolet (UV) rays to support endogenous production (6).

Maternal folate deficiency can have detrimental effects on fetal development, increasing the risk of neural tube defects (NTDs), including spina bifida and anencephaly (29). An adequate intake of folic acid successfully prevents NTDs; however, as pregnant women are unlikely to meet their folic acid requirements through diet alone, supplementation of 400 µg per day is recommended for all women preconception and in their first 12 weeks of pregnancy (30). Maternal obesity is a risk factor for NTDs, and as obese women may have lower serum vitamin B12 and folate levels during pregnancy compared to mothers with a healthy weight, they may need even greater folic acid supplementation as well as vitamin B12 (16,31). In addition, megaloblastic anemia can occur because of prolonged inadequate folic acid or vitamin B12 intake (32).

Women who enter pregnancy with low iodine stores are at risk of hypothyroidism that is associated with neurodevelopmental delay in offspring (6). Insufficient intakes have been reported in pregnant populations (33–35). Therefore, the need for supplementation should be assessed early in pregnancy, especially in areas where noniodized salt is consumed, where there is low intake of seafood or dairy products, or in locations where there is known iodine deficiency (6). In addition, selenium is essential for fetal growth, thyroid metabolism, and the prevention of oxidative stress. Insufficiency may be associated with preeclampsia and gestational diabetes mellitus (GDM) as well as loss of pregnancy (36).

Women with protein-energy malnutrition or poor dietary quality may be at risk of zinc deficiency, and zinc requirements increase by up to 40% in pregnancy. Zinc is essential for fetal growth, immune function, and neurological development. Supplementation in high-risk populations has been shown to reduce the risk of preterm birth and increase birth weight (37). In certain countries, vitamin A deficiency affects pregnant women and is associated with abnormal fetal development, preterm birth, and maternal mortality, among other outcomes. Conversely, vitamin A is teratogenic (e.g., increasing risk of craniofacial and heart defects), and in areas where vitamin A deficiency is uncommon, excessive consumption should be avoided through limiting rich sources such as liver and using pregnancy-specific micronutrient supplements (6).

Maternal weight and gestational weight gain

Body mass index (BMI) during early pregnancy can be used to identify women who are at increased risk of pregnancy complications such as GDM, preeclampsia, and cesarean section. Maternal BMI is closely associated with intrauterine growth and birth weight. Up to a quarter of pregnant women in the United Kingdom are obese at conception, and the prevalence of obesity is rising worldwide (6,38). Obesity and excessive GWG increase the risk of delivering large for gestational age infants, which increases the risk of delivery complications (39). In addition to macrosomia, maternal obesity increases the risk of adverse outcomes for the fetus, including congenital abnormalities and preterm birth. Conversely, women who are undernourished during pregnancy have an increased risk of delivering a small for gestational age infant. They also have diminished energy reserves and may have suboptimal status of one or more key micronutrients needed to support healthy fetal development (6). In fact, women who enter pregnancy with an underweight or overweight BMI have higher incidence of spontaneous abortion (40). Therefore, women should have their BMI calculated at their first antenatal visit and be provided with advice on appropriate GWG rather than weight loss.

While there is no international consensus on appropriate GWG, the Institute of Medicine guidelines are commonly used; however, these are based on prospective studies in high-income countries, so they may not be applicable to all women (6). In the first trimester of pregnancy, very modest weight gain is expected, which may range between 1 and 3 kg for all women except those who are obese, in which case a smaller weight gain of 0.2–2 kg is recommended (41). As outlined in Table 2.2, the bulk of GWG is expected in the second and third trimesters, and levels of GWG vary depending on maternal pre-pregnancy BMI. Women who gain weight within these ranges have better pregnancy outcomes; however, most women gain weight outside of these recommendations (42). A recent Cochrane review found high-quality evidence that dietary intervention (with or without exercise) can reduce the risk of excessive GWG. Dietary interventions include low glycemic index diet, energy restriction, or reducing fat (43). Alternatively, to encourage sufficient weight gain in mothers who are malnourished, antenatal nutritional education to increase energy and protein intake can be provided and has been shown to reduce the risk of preterm birth and LBW (44).

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