The Food Standards Agency has a really useful page with all the official advice and guidelines: here
It's main advice is:
"It's important to try to eat a variety of foods including:
* plenty of fruit and vegetables (fresh, frozen, tinned, dried or a glass of juice). Aim for at least five portions of a variety each day
* plenty of starchy foods such as bread, pasta, rice and potatoes - try to choose wholegrain options
* foods rich in protein such as lean meat and chicken, fish (aim for at least two servings of fish a week, including one of oily fish), eggs and pulses (such as beans and lentils). These foods are also good sources of iron (see 'Do I need extra iron?' below)
* plenty of fibre. This helps prevent constipation and is found in wholegrain bread, pasta, rice, pulses and fruit and vegetables
* dairy foods such as milk, cheese and yoghurt, which contain calcium
It's also a good idea to cut down on foods such as cakes and biscuits, because these are high in fat and sugar. This can also help you to avoid putting on too much weight during pregnancy.
Healthy snacks to have instead include malt loaf; currant buns without icing; sandwiches or pitta bread filled with cottage cheese, chicken or lean ham; low-fat yoghurts; vegetable and bean soups; and fruit including fresh, tinned in juice or dried fruit such as raisins or apricots."