Pregnancy is one of the most nutritionally demanding periods of a woman's life — not because she needs to eat dramatically more, but because the quality of what she eats directly shapes the development of a new human being. The "eating for two" cliché is misleading: calorie requirements increase by only around 200–300kcal in the third trimester. But micronutrient requirements increase significantly, and the consequences of deficiency during critical developmental windows are serious and sometimes permanent.
The First Trimester: Foundations
The first 12 weeks are when the neural tube — which becomes the brain and spinal cord — forms and closes. This process happens between weeks three and six, often before a woman knows she is pregnant. This is why folate (folic acid) supplementation is recommended for all women who might become pregnant or who are trying to conceive, not just those who have confirmed pregnancy.
Neural tube defects including spina bifida are significantly more common in folate-deficient pregnancies. The UK NHS recommends 400mcg of folic acid daily from pre-conception until 12 weeks. Women with a family history of neural tube defects, those taking anti-epileptic medications, and those with BMI over 30 or diabetes are advised to take the higher dose of 5mg, prescribed by a GP.
Food sources of folate (the natural form) include dark leafy greens (spinach, kale, broccoli), beans and lentils, asparagus, avocado, and citrus fruits. Fortified foods provide folic acid (the synthetic form, which is more bioavailable). Both contribute, but supplementation during the critical early period is non-negotiable.
Iron: The Most Common Deficiency in Pregnancy
Blood volume increases by approximately 50% during pregnancy, and this expanded volume requires substantially more haemoglobin — the iron-containing protein that carries oxygen. Iron requirements in pregnancy are approximately double those of non-pregnant women. Iron deficiency anaemia during pregnancy is associated with increased risk of preterm birth, low birth weight, and postnatal depression, as well as fatigue, breathlessness, and impaired immune function in the mother.
The best sources of dietary iron are haem iron from meat and fish, which is absorbed more efficiently than non-haem iron from plant sources. Red meat two to three times per week, combined with liver once per week if tolerated (noting that liver is very high in vitamin A, which should not be consumed in excess during pregnancy — once weekly is safe), provides substantial iron. For vegetarians, lentils, chickpeas, kidney beans, tofu, dark leafy greens, fortified cereals, and pumpkin seeds are all valuable. Consuming vitamin C alongside non-haem iron sources (a glass of orange juice with an iron-rich meal, or a squeeze of lemon on beans) significantly increases absorption. Avoid tea and coffee with iron-rich meals — tannins inhibit iron absorption considerably.
Routine iron testing at antenatal appointments identifies deficiency. If diagnosed with anaemia, iron supplements are usually prescribed — oral ferrous sulfate is effective but often poorly tolerated. Taking with food and using a different form (ferrous gluconate or fumarate) reduces gastrointestinal side effects for most women.
Calcium and Vitamin D: Building Bones
The baby's skeleton requires substantial calcium throughout pregnancy, particularly in the third trimester when bone mineralisation accelerates. If maternal calcium intake is insufficient, the baby draws from maternal bone stores — a process that can compromise long-term maternal bone density. UK dietary guidelines recommend 700mg daily for pregnant women, achieved through dairy products, fortified plant milks, canned fish with bones (sardines, salmon), calcium-set tofu, and dark leafy greens.
Vitamin D is essential for calcium absorption and bone mineralisation. The UK's limited sunshine and indoor lifestyles mean vitamin D deficiency is extremely common. During pregnancy, deficiency is associated with increased risk of pre-eclampsia, gestational diabetes, preterm birth, and low birthweight. The NHS recommends all pregnant women supplement with 10 micrograms (400 IU) of vitamin D3 daily throughout pregnancy and breastfeeding.
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Omega-3 DHA: Brain and Eye Development
DHA (docosahexaenoic acid), an omega-3 fatty acid, is critical for foetal brain and retinal development. The foetal brain accumulates DHA rapidly during the third trimester and the first two years of life. Maternal DHA status directly determines how much is available for the developing baby. Low maternal DHA intake is associated with slower neurodevelopmental outcomes in the child.
Oily fish — salmon, mackerel, sardines, trout — are the best dietary source of DHA. The NHS recommends two portions of fish per week during pregnancy, with a maximum of two portions of oily fish due to potential trace contaminants (mercury, PCBs). Swordfish, shark, and marlin should be avoided entirely due to high mercury content. Tuna should be limited to four medium cans or two fresh steaks per week. Vegetarians and vegans should consider algae-based DHA supplements, which provide DHA without the fish.
Foods to Avoid During Pregnancy
Several foods carry risks during pregnancy that are either absent or much lower outside pregnancy. Soft mould-ripened cheeses (brie, camembert, soft blue cheeses) carry Listeria risk; hard cheeses and pasteurised soft cheeses are safe. Raw or undercooked meat, poultry, and fish risk Toxoplasma and Salmonella. Raw eggs and products made with them (some mayonnaises, mousses, ice creams) risk Salmonella — lion-marked UK eggs are produced from vaccinated hens and are safe to eat runny. Liver and liver products should be limited to once weekly due to high vitamin A content. Unpasteurised milk and juice carry bacterial risks. Alcohol should be avoided entirely throughout pregnancy.
Caffeine should be limited to 200mg daily (roughly two cups of coffee or four cups of tea). Higher intakes are associated with increased miscarriage risk and lower birthweight.
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