The fields of obstetrics and nutrition have changed over the last few decades, greatly affecting recommendations for prenatal nutrition. The importance of nutrition in pregnancy cannot be overstated. It maintains maternal energy requirements, provides substrate for the development of new fetal tissues, and builds energy reserves for postpartum lactation. Recommendations for prenatal nutrition typically focus on weight gain and dietary intake in pregnancy.
Weight Gain and Pregnancy
An infant's birth weight is affected by many factors, including the mother's pre-pregnancy Body Mass Index (BMI) and weight gain during pregnancy. BMI is defined as weight in kilograms divided by the square of height in meters.
Underweight is defined as a BMI of less than 18.5 kg/m2. Normal weight is defined as a BMI of 18.5-24.9 kg/m2. Overweight is defined as a BMI of 25-29.9 kg/m2. Obesity is defined as a BMI that exceeds 30 kg/m2.  Birth weight is affected by pre-pregnancy BMI, independent of actual weight gain during pregnancy. Women who are underweight are at increased risk for low birth weight babies; women who are overweight or obese are at increased risk for macrosomic infants. Macrosomic infants are at increased risk for shoulder dystocia and brachial plexus injuries.
Morbid obesity is defined by a BMI exceeding 35 kg/m2. Morbidly obese pregnant patients are at increased risk for preeclampsia, nonreassuring fetal heart tracings, meconium aspiration, late intrauterine fetal death, and early neonatal death. 
Birth weight is also affected by maternal weight gain during pregnancy. Although weight should be gained throughout pregnancy, it is most critical in the second trimester. Even if overall weight gain is poor, birth weight is usually acceptable as long as second-trimester weight gain is appropriate. 
The Institute of Medicine’s 2009 pregnancy weight gain recommendation guidelines for singleton pregnancies are  :
§ Underweight - 28-40 lbs
§ Normal weight - 25-35 lbs
§ Overweight - 15-25 lbs
§ Obese - 11-20 lbs
Dieting to reduce weight during pregnancy is never recommended, even for patients who are morbidly obese. Medications for weight management are also not recommended during time of conception and pregnancy, however data supports that pharmacotherapy and bariatric surgery are safe before pregnancy, most literature suggesting 12-18 months prior to conception. [1, 4] Because of the expansion of maternal blood volume and construction of fetal and placental tissues, some weight gain is essential for a healthy pregnancy.
Weight gain within the IOM parameters is associated with a lower rate of cesarean delivery, fewer infants with growth restriction or macrosomia, and a decreased incidence of postpartum obesity. Only 30-40% of pregnant women achieve appropriate weight gain.  Further evaluation is needed if weight gain is persistently slow or less than 10 lb by mid pregnancy.
Diet in Pregnancy
The metabolic demands of pregnancy require supplemental dietary intake. Additional energy (caloric) intake is required to support recommended weight gain. Because energy requirements in pregnancy are increased by 17% over the nonpregnant state, a woman of normal weight should consume an additional 300 kcal/d. This energy should be of high nutrient density. Nutrient density reflects the amount of protein, vitamins, and minerals per 100 kcal of food. 
Protein should comprise 20% of a normal pregnancy diet. The recommended daily allowance (RDA) in pregnancy is 60g. Most American diets already contain more than enough protein. Pregnant women should be counseled that many animal sources of protein are very high in fat and might contribute to excessive weight gain, so should be taken sparingly. Fat should only comprise 30% of a normal pregnancy diet. Carbohydrates should comprise the remaining 50%.
A sample diet for normal pregnancy is based on Myplate recommendations and is tailored to each woman based on their height, weight, activity level and trimester of pregnancy.  The new recommendations regarding energy intake focus on trimester specific needs. It is now recommended that women maintain their non-pregnancy intake in the first trimester and increase by 340kcal/d and 452 kcal/d in the second and third trimesters. Baseline estimated energy requirements (EER) can be estimated based on this equation: EER = 354 − (6.91 × age [y]) + PA × [(9.36 × weight [kg]) + (726 v height [m])]. 
Vitamins in Pregnancy
Vitamin A, a fat-soluble vitamin, is important for maintenance of maternal visual function. Its main influence is on the retina, but it also aids glycoprotein synthesis and promotes cellular growth and differentiation in other tissues. Vitamin A is found in green leafy vegetables and yellow-orange vegetables. The nonpregnancy RDA is 700 mcg, and the pregnancy RDA is 770 mcg. The lactation RDA is 1300 mcg. Well-balanced diets provide the RDA for women who are pregnant or lactating, so, routine supplementation is not recommended. Doses exceeding 15,000 IU/d, often used to treat acne, are associated with an increased risk of birth defects and should not be used in pregnancy. Alpha-carotene, a vitamin A precursor, is not teratogenic.
Vitamin B-1, also known as thiamine, is a water-soluble B-complex vitamin. It is involved in carbohydrate metabolism. Its food sources include milk and raw grains. The RDA is 1.1 mg. In both pregnancy and lactation, the RDA increases to 1.4 mg. Well-balanced diets provide the pregnant and lactating RDA. Routine supplementation is not recommended. Supplementation may need to be considered in patients with hyperemesis. 
Vitamin B-2, also known as riboflavin, is a water-soluble B-complex vitamin. It is also involved in the release of energy from cells. Vitamin B-2 is found in green vegetables, milk, eggs, cheese, and fish. The RDA is 1.1 mg. In pregnancy, the RDA increases to 1.4 mg; in lactation, it increases to 1.6 mg. Well-balanced diets provide the RDA for women who are pregnant or lactating. Routine supplementation is not recommended.
Vitamin B-6, also known as pyridoxine, is a water-soluble B-complex vitamin. It is important in protein, carbohydrate, and lipid metabolism. It is also involved in the synthesis of red blood cells. Vitamin B-6 is found mostly in vegetables. The RDA is 1.2-1.5 mg. The pregnancy RDA is 1.9 and in lactation, the RDA increases to 2 mg. Well-balanced diets provide the pregnant and lactating RDA. Routine supplementation is not recommended.
Vitamin B-12, also known as cobalamin, a water-soluble B-complex vitamin, is essential for DNA synthesis and cell division making it key for embryogenesis. It helps form red blood cells and maintains the nervous system.  It is found in animal proteins. Deficiency is usually secondary to compromised intestinal function. Dietary deficiency is rare, but is occasionally encountered in persons who follow strict vegan diets. The RDA is 2.4 mcg. The pregnancy RDA is 2.6 mcg. The RDA increases to 2.8 mcg with lactation. Well-balanced diets provide the RDA for women who are pregnant or lactating. Routine supplementation is not recommended.
Vitamin C, also known as ascorbic acid, is a water-soluble vitamin with numerous functions including reducing free radicals and assisting in procollagen formation and the absorption of iron.  Vitamin C is found in fruits and vegetables. Chronic deficiency impairs collagen synthesis and leads to scurvy. The RDA is 75 mg. The pregnancy RDA is 85 mg. The RDA increases to 120 mg with lactation. Well-balanced diets provide the RDA for women who are pregnant or lactating. Routine supplementation is not recommended.
Vitamin D, a fat-soluble vitamin, is found in fortified milk. Exposure to ultraviolet light is necessary for vitamin conversion. Deficiency of vitamin D is associated with tooth enamel hypoplasia. Vitamin D is also responsible for the promotion of healthy bone growth eyesight and skin in the fetus. [6, 11] The RDA in both pregnancy and lactation is 5 mcg. Well-balanced diets provide the RDA for women who are pregnant or lactating. Routine supplementation is not recommended.
In 2011, ACOG reported insufficient evidence to support a recommendation for screening all pregnant women for vitamin D deficiency.  Routine screening for vitamin D levels is not recommended in the United States.
Vitamin E, a fat-soluble vitamin, is an important antioxidant. It is found in animal protein and fats. The RDA is 15 mg. The pregnancy RDA is 15 mg; the lactation RDA is 19 mg. Well-balanced diets provide the RDA for women who are pregnant or lactating. Routine supplementation is not recommended. 
Vitamin K, a fat-soluble vitamin, is required for synthesis of clotting factors VII, IX, and X and therefore essential for normal coagulation. It is found in green leafy vegetables, tomatoes, dairy products, and eggs. There is limited maternal to placental transfer during gestation. The American Academy of Pediatrics recommends that neonates be given intramuscular supplementation at birth. [16, 17] The RDA is 90 mg. In pregnancy and lactation, the RDA stays at 90 mg. Well-balanced diets provide the RDA for women who are pregnant or lactating. Routine supplementation is not recommended.
Folic acid, a water-soluble B-complex vitamin, is important for DNA synthesis and cell replication. It is found in fortified grains, dried beans, and leafy greens.  Deficiency in pregnancy has been linked with maternal megaloblastic anemia and fetal neural tube defects. The RDA is 0.4 mg. The RDA is 0.6 mg during pregnancy and 0.5 mg during lactation.
In 1998, the US Food and Drug Administration (FDA) mandated fortification of grains with folate. The degree of fortification was calculated to provide only 0.1 mg/d of dietary folate. Fortification was enacted to avoid having supplemental folate mask evidence of vitamin B-12 deficiency in susceptible populations, especially in the elderly. Cereal fortification has resulted in a 32% decrease in the prevalence of elevated maternal serum alpha-fetoprotein values  and a 25% decline in the prevalence of open neural tube defects.  Most recently, the FDA extended this fortification to corn masa flour given the incidence of neural tube defects in the Hispanic American population. 
Despite fortification, the average American diet has insufficient folic acid and routine supplementation of 0.4 mg/d is recommended for healthy women. Folate supplements should be administered 3 months prior to conception and throughout the first trimester. If the mother had a prior child affected by a neural tube defect, supplementation in the subsequent pregnancy should be increased to 4 mg/d. 
Niacin is a water-soluble vitamin involved in the release of energy from cells. It is found in poultry, fish, and nuts. Deficiency results in pellagra. The RDA is 14 mcg. In pregnancy, the RDA increases to 18 mcg, and to 17 mcg in lactation. Well-balanced diets provide the RDA for women who are pregnant and lactating. Routine supplementation is not recommended.
Minerals and Trace Elements in Pregnancy
The US recommended daily allowances (RDA) for iodine intake are 150 μg in adults, 220-250 μg in pregnant women, and 250-290 μg in breastfeeding women. Dietary sources such as iodized salt, dairy products, some breads, and seafood usually contain enough to meet the RDA for most people who are not pregnant or lactating. Iodine is essential for healthy brain development in the fetus and young child and is crucial for thyroid hormone production.  Iodine deficiency results in maternal and fetal goiter, cretinism, intellectual impairment and neonatal hypothyroidism. It is also associated with an increased risk of pregnancy loss. 
Iron is essential for the production of hemoglobin. Its dietary sources include animal protein, dried beans, fortified grains, and any food cooked in cast iron cookware. Despite its numerous sources, women have difficulty maintaining iron balance through diet alone. A well-balanced diet with 2500 kcal usually contains approximately 15 mg of elemental iron. However, the absorption of iron is very inefficient and only approximately 10% is absorbed. With each normal menses, 12-15 mg of elemental iron is lost. It is estimated that a woman's diet must include 1.5-2 mg/d of elemental iron to compensate for menstrual losses alone. In pregnancy, 500 mg of additional iron is needed to expand the maternal red cell mass. Another 500 mg is needed to supply fetal and placental tissues. On average, an additional 3 mg/d of elemental iron must be absorbed from dietary sources during pregnancy.
The RDA for elemental iron reflects these increased requirements. The RDA for women who are not pregnant is 15 mg, 1.5 mg of which is absorbed. In pregnancy, the RDA is 30 mg, 3 mg of which is absorbed. During lactation, the RDA returns to 15 mg. Well-balanced diets do not provide the pregnancy RDA for elemental iron. Iron supplementation is recommended in pregnancy.
Various iron preparations are commercially available, and each delivers a slightly different amount of elemental iron. These preparations include ferrous sulfate, ferrous fumarate, ferrous gluconate, and polysaccharide iron complex. Pure elemental iron is available in 50-mg caplets of carbonyl iron. Providers should be aware of the elemental iron contained in any one specific preparation, and that only 10% of this is absorbed from the maternal gut. Absorption is enhanced by concurrent ingestion of foods containing vitamin C. Usually, one dose of any preparation containing at least 30 mg of elemental iron meets the RDA. Larger doses are required only to treat maternal iron deficiency anemia and only serve to constipate patients without anemia. Iron competes with zinc at absorption sites; the clinical implications of this are discussed below in the zinc section.
Iron deficiency anemia is one of the most common pregnancy complications. Screening for iron deficiency anemia is recommended at the first prenatal visit and thereafter as indicated. Iron deficiency anemia is suggested if the complete blood cell count suggests a microcytic, hypochromic anemia. Confirmatory test results include a reduced serum iron level, increased total iron-binding capacity, decreased transferrin saturation, and reduced serum ferritin levels. If a provider is unable to perform the complete battery of confirmatory tests, the serum iron and ferritin levels usually suggest the correct diagnosis. Treatment is increased oral iron supplementation.
High hemoglobin values have been associated with adverse pregnancy outcomes; however, iron supplementation cannot, by itself, raise hemoglobin to these levels.  These adverse outcomes are more likely secondary to the underlying conditions responsible for the high hemoglobin values.
Calcium is a major component of bone. Large quantities of calcium are required in pregnancy for construction of fetal tissues, especially in the third trimester. Pregnant women younger than 25 years also still require calcium for their own bone mass. Hormonal adaptations and increased intestinal absorption protect maternal bone while meeting fetal calcium requirements. A well-balanced diet provides adequate calcium to meet all of these needs, and supplementation is not recommended. The RDA for nonpregnant, pregnant, and lactating women is 1000 mg. 
Calcium is found in dairy products and leafy green vegetables such as collard, kale, turnip, and mustard greens. Vitamin D is required for calcium absorption.
Along with calcium, phosphorus is required for bone formation. Maternal serum inorganic phosphorus levels remain constant during pregnancy because of maternal adaptations. The RDA for nonpregnant, pregnant, and lactating women is 700 mg. Well-balanced diets easily provide the RDA for nonpregnant, pregnant, and lactating women. Supplementation is not recommended. Phosphorus is not usually present in vitamin supplements.
Zinc is involved in nucleic acid and protein metabolism, and is important in early gestation for growth and development. The RDA is 8 mg. The RDA for pregnant women is 11 mg, which increases to 12 mg during lactation. Well-balanced diets provide the RDA for women who are pregnant and lactating, and supplementation is not recommended. Both iron and copper compete with zinc at absorption sites. Zinc supplementation is recommended when elemental iron supplementation exceeds 60 mg/d. Likewise, whenever zinc supplements are used, copper should also be supplemented. Different prenatal vitamin formulations contain different amounts of copper and zinc. Usually, copper or zinc supplementation can be accomplished by appropriate selection of a prenatal vitamin formulation. 
Sodium is present in large quantities in the average American diet. The RDA is 1.5 mg during pregnancy, lactation, and the non-pregnant state. Whether pregnant or not, sodium should neither be restricted nor used excessively.  Well-balanced diets "salted to taste" satisfy sodium requirements and obviate any need for supplementation. Pregnant women should be counseled that most processed and pre-prepared foods are high in sodium. 
Prenatal Vitamin Supplements
A standard prenatal vitamin formulation contains:
Iron - 30 mg
Zinc - 15 mg
Calcium - 250 mg
Vitamin B-6 - 2 mg
Folate - 0.4 mg
Vitamin C - 50 mg
Vitamin D - 5 mcg
Contents vary by individual formulation, and nutrient supplementation should be chosen with attention to individual patient needs. A normal well-balanced pregnancy diet generally provides the RDA of all nutrients except elemental iron and folate, both of which must be supplemented. Prenatal vitamins are not necessary for every patient. However, these supplements are routinely prescribed to most patients. A major reason is that a nutritionally compromised pregnancy can be difficult to identify, and the potential benefits of routine supplementation overshadow any risk from prenatal vitamin supplements.
A systematic review of prenatal multimicronutrient supplementation on pregnancy outcomes showed a significant reduction in risk of low birth weight for mothers taking multimicronutrients compared with those taking with placebo or iron/folic acid supplementation. Birth weight was significantly higher in newborns in the multimicronutrient group compared with the iron/folic acid supplementation group. No significant differences were noted between multimicronutrients, iron/folic acid supplementation, or placebo on the risk of preterm birth or small-for-gestational-age infants. 
A randomized controlled trial studied the effect of maternally administered vitamins C and E on risk of spontaneous preterm birth in low-risk, nulliparous women.  Study participants were given either vitamin C 1000 mg and vitamin E 500 IU or placebo starting between 9 and 16 weeks' gestation and continuing until delivery. Supplementation did not reduce spontaneous preterm births in this population.
Calcium can be obtained from lactose-free dairy products or calcium-enriched orange juice or soy milk. If the daily calcium intake is less than 600 mg (one serving of calcium-rich food), patients will benefit from calcium supplementation.  The total daily divided dose should be 500-1000 mg. The maximum tolerable daily divided dose is 2500 mg. If a woman is unable to tolerate any dairy products and has limited exposure to sunlight, she may also require supplemental vitamin D (400 IU/d). 
Vegetarian and vegan diets
Vegetarian diets with multiple variations are becoming increasingly common. Well-balanced vegetarian diets that include dairy products provide adequate energy and nutrient intake and do not require special supplementation. Vegan diets include no animal products whatsoever, including, meat, dairy, and eggs. A vegan diet, even if well balanced in all other respects, may be deficient in calcium, vitamins D and B-12, and essential fatty acids. These deficiencies can be resolved with fortified food products. 
The pregnant adolescent may also require careful attention to energy intake. Younger women may have increased energy requirements to satisfy the demands of ongoing maternal growth. Special attention should be taken with the adolescent population to focus on food rather than nutrition and to have an individualized approach regarding their eating habits. Consultation with nutritionists and counselors may be helpful if these dietary issues arise. 
Women with multiple gestations have increased nutritional requirements. The recommended weight gain for twin gestations is 16.8-24.5 kg for normal BMI, 14.1 – 22.7 kg for overweight patients, and 11.3-19.1 for obese patients.  This additional weight gain requires approximately 150 kcal/d over the dietary requirements of singleton pregnancies. Women with triplets should gain 50 lb.  Nutrient requirements are also increased in multiple gestations. Routine prenatal vitamin and mineral supplementation is recommended. The suggested amount of folic acid supplementation is 1 mg/d. Elemental iron requirements are increased, often requiring more frequent doses of iron supplements. Lastly, vitamin B-6 requirements are increased. The recommended supplementation for women with multiple pregnancies is 2 mg/d. 
Hyperemesis gravidarum is a common complication of early pregnancy. Mild hyperemesis in the first trimester, when weight gain is not yet essential to fetal growth, is unlikely to affect fetal growth or development. Initiation of prenatal vitamins three months prior to conception appears to reduce the severity and frequency of pregnancy associated nausea and vomiting.  Frequent, small meals and snacks as opposed to large food boluses are thought to reduce symptoms. If this fails, patients may respond to vitamin B6, 25 mg 3 times a day or vitamin B6 plus doxylamine.  When hyperemesis precludes all oral intake, severe dehydration and ketosis may result, which requires inpatient management and intravenous rehydration. 
Eating disorders are common in women during their reproductive years. Anorexia nervosa commonly results in amenorrhea and is not often observed in pregnancy. However, bulimia may complicate pregnancy. Unless severe, bulimia is unlikely to affect fetal development in the first trimester. If it becomes persistent, the severe energy restriction may compromise fetal growth. Providers should treat the disorder before the pregnancy and should provide general nutrition advice. Psychiatric and dietician referrals should be considered. 
Malabsorption syndromes and inflammatory bowel disease may compromise fetal growth. Increased ultrasonographic surveillance may aid in early detection of a growth-impaired pregnancy. Special diets and supplements may be required. These patients should be treated in conjunction with a gastroenterologist.
The use of bariatric surgery has increased dramatically over the last 20 years, leading to an increased number of pregnancies following bariatric/gastric bypass surgery.
Several types of procedures are performed. According to the American Society for Metabolic and Bariatric Surgery, the most common bariatric surgery procedures are gastric bypass (most frequency the roux-en-Y or RGB) (23.1%), sleeve gastrectomy (53.8%), adjustable gastric band (5.7%) and the biliopancreatic diversion with duodenal switch (0.6%).  Most procedures are performed laparoscopically. In general, gastric restriction limits the amount of food eaten, causing early satiety by decreasing the size of the stomach and outlet. The RGB has 2 components, gastric restriction and bypassing the duodenum and directly connecting the stomach to the jejunum, thus creating early satiety, mild malabsorption, and causing dumping syndrome when a large sugar load is ingested. The BPD combines gastric restriction and a greater degree of malabsorption.
Nutritional complications are not seen in purely restrictive procedures. Malabsorption procedures cause most of the nutritional complications seen in patients undergoing bariatric surgery. Patients undergoing BPD and RGB often develop protein malnutrition, anemia, and deficiencies of iron, folate, calcium, vitamin B-12, and the lipid soluble vitamins such vitamins D, K, E and A.
No consistent recommendations have been made regarding the frequency of monitoring nutritional deficiencies in patients who become pregnant after undergoing bariatric surgery. In a recent review of bariatric surgery, patients were advised to not get pregnant during the more rapid phase of weight loss because of an increased risk for micronutrient and macronutrient deficiencies during this period.  Bariatric surgeons have a wide range of practice for monitoring micronutrient and macronutrient deficiencies.
The American College of Obstetrics and Gynecology recommends that all patients have preconception counseling and prenatal assessment of their nutritional status and, where appropriate, replacement of vitamin B-12, folic acid, iron, and calcium [Table 1]. [35, 4] The assessment of other potential nutritional deficiencies should be based on the practitioner’s clinical judgment. Pregnancy outcomes are favorable overall. Pregnancies after bariatric surgery can be associated with adverse perinatal outcomes, but are less likely to be complicated by gestational diabetes, hypertension, macrosomia, and cesarean delivery than are pregnancies of obese women who have not had the surgery.  Consultation with a perinatologist may be helpful.
Poverty nearly doubles the rate of insufficient weight gain. Nutritional counseling may assist patients who are impoverished in identifying low-cost foods with high-nutrient density. The federal Women, Infants, and Children (WIC) program, administered through the US Department of Agriculture Food and Nutrition Service, is a valuable resource for women who are impoverished (see Food and Nutrition Service).
In the United States, the vast majority of pregnant women enjoy good nutrition. However, in much of the developing world, pregnant women may experience nutritional deficiencies not commonly observed in the United States.
Micronutrient supplementation may be beneficial to the maternal-child health of developing nations.  The increased serum hemoglobin level associated with iron supplementation can result in a greater attack rate of clinical malaria. Malarial prophylaxis is indicated when large doses of elemental iron are given to pregnant women in endemic areas.