Nutrition in pregnancy

SUMMARY INTRODUCTION PHYSIOLOGICAL CHANGES DURING PREGNANCY Changes in body composition and weight gain Changes in blood composition Metabolic changes and adaptive responses Key points BIRTHWEIGHT AND THE FETAL ORIGINS HYPOTHESIS Factors associated with birthweight The fetal origins of adult disease hypothesis Key points ESSENTIAL FATTY ACIDS AND PREGNANCY Key points PRE‐PREGNANCY NUTRITIONAL ISSUES Bodyweight and fertility Pre‐pregnancy weight and birth outcome Nutritional status Folate/folic acid Key points NUTRITIONAL REQUIREMENTS DURING PREGNANCY Energy Protein Fat Carbohydrate Vitamins Vitamin A Thiamin, riboflavin and folate Vitamin C Vitamin D Minerals Calcium Iron Key points FOOD SAFETY ISSUES DURING PREGNANCY Vitamin A Alcohol Caffeine Foodborne illness Listeriosis Salmonella Toxoplasmosis Campylobacter Fish Food allergy Key points DIET‐RELATED CONDITIONS DURING PREGNANCY Nausea and vomiting and changes in taste and appetite Constipation Anaemia Gestational diabetes Hypertensive disorders Key points ISSUES FOR SPECIFIC GROUPS Vegetarians and vegans Teenage pregnancy Dieting during pregnancy Key points CONCLUSIONS AND RECOMMENDATIONS FURTHER INFORMATION REFERENCES Summary A healthy and varied diet is important at all times in life, but particularly so during pregnancy. The maternal diet must provide sufficient energy and nutrients to meet the mother's usual requirements, as well as the needs of the growing fetus, and enable the mother to lay down stores of nutrients required for fetal development as well as for lactation. The dietary recommendations for pregnant women are actually very similar to those for other adults, but with a few notable exceptions. The main recommendation is to follow a healthy, balanced diet based on the Balance of Good Health model. In particular, pregnant women should try to consume plenty of iron‐ and folate‐rich foods, and a daily supplement of vitamin D (10 µg/day) is recommended throughout pregnancy. There are currently no official recommendations for weight gain during pregnancy in the UK. For women with a healthy pre‐pregnancy weight, an average weight gain of 12 kg (range 10–14 kg) has been shown to be associated with the lowest risk of complications during pregnancy and labour, and with a reduced risk of having a low birthweight (LBW) infant. However, in practice, well‐nourished women with a normal pre‐pregnancy bodyweight show wide variations in weight gain during pregnancy. Low gestational weight gain increases the risk of having a LBW infant, whereas excessive weight gain during pregnancy increases the risk of overweight and obesity in the mother after the birth. A birthweight of 3.1–3.6 kg has been shown to be associated with optimal maternal and fetal outcomes for a full‐term infant. LBW (birthweight < 2.5 kg) is associated with increased infant morbidity and mortality, as well as an increased risk of adult diseases in later life, such as cardiovascular disease and type 2 diabetes. The fetal origins hypothesis states that chronic diseases in adulthood may be a consequence of ‘fetal programming’, whereby a stimulus or insult at a critical, sensitive period in development has a permanent effect on structure, physiology or function. However, there is little evidence that in healthy, well‐nourished women, the diet can be manipulated in order to prevent LBW and the risk of chronic diseases in later life. Maternal nutritional status at the time of conception is an important determinant of fetal growth and development, and therefore a healthy, balanced diet is important before, as well as during, pregnancy. It is also important to try and attain a healthy bodyweight prior to conception body mass index (BMI) of 20–25, as being either underweight or overweight can affect both fertility and birth outcome. It is now well recognised that taking folic acid during the peri‐conceptional period can reduce the incidence of neural tube defects (NTDs), and all women who may become pregnant are advised to take a folic acid supplement of 400 µg/day prior to and up until the 12th week of pregnancy. The UK Committee on Medical Aspects of Food Policy (COMA) panel has established dietary reference values (DRVs) for nutrients for which there is an increased requirement during pregnancy. This includes thiamin, riboflavin, folate and vitamins A, C and D, as well as energy and protein. The energy costs of pregnancy have been estimated at around 321 MJ (77 000 kcal), based on theoretical calculations and data from longitudinal studies. In practice, individual women vary widely in metabolic rate, fat deposition and physical activity level, so there are wide variations in individual energy requirements during pregnancy. In the UK, the recommendation is that an extra 200 kcal of energy per day is required during the third trimester only. However, this assumes a reduction in physical activity level during pregnancy, and women who are underweight or who do not reduce their activity level may require more. The COMA DRV panel did not establish any increment in requirements for any minerals during pregnancy, as physiological adaptations are thought to help meet the increased demand for minerals, e.g. there is an increase in absorption of calcium and iron. However, certain individuals will require more calcium, particularly teenagers whose skeletons are still developing. Up to 50% of women of childbearing age in the UK have low iron stores, and are therefore at risk of developing anaemia should they become pregnant. Moreover, around 40% of women aged 19–34 years currently have an iron intake below the lower reference nutrient intake (LRNI). Pregnant women are therefore advised to consume plenty of iron‐rich foods during pregnancy and, in some cases, supplementation may be necessary. There are a number of food safety issues that apply to women before and during pregnancy. It is advisable to pay particular attention to food hygiene during pregnancy, and to avoid certain foods ( e.g. mould‐ripened and blue‐veined cheeses) in order to reduce the risk of exposure to potentially harmful food pathogens, such as listeria and salmonella. Pregnant women, and those who may become pregnant, are also advised to avoid foods that are high in retinol ( e.g. liver and liver products), as excessive intakes are toxic to the developing fetus. It is also reco