Folic Acid

Folic acid is very important for the development of a healthy foetus, as it can significantly reduce the risk of neural tube defects (NTDs), such as spina bifida. It is also known as vitamin B9.
The UK Department of Health recommends that women should take a daily supplement of 0.4mg of folic acid while they are trying to conceive, and should continue taking this dose of folic acid for the first 12 weeks of pregnancy, when the baby's spine is developing.
Dietary sources of folic acid include green, leafy vegetables, breakfast cereals and granary bread. Always check the food labels. However, it would be almost impossible to get enough folic acid just from food; the only way to be sure you are getting the right amount is by taking a supplement.
Liver is also very rich in folic acid, although it is not safe to eat while you are pregnant, or trying to become pregnant. This is because liver is also very rich in vitamin A, too much of which can cause birth defects in your baby.
Some women have an increased risk of having a baby with a neural tube defect (NTD). You may have a higher than average risk if:

you have previously given birth to a baby with an NTD,
you are taking an antiepileptic medication,
either you, or your partner, has an NTD,
you have a condition caused by a sensitivity to gluten (coeliac disease), or
you have diabetes.

If any of the above applies to you, a higher dose of folic acid (5.0 mg) is recommended to reduce your chances of having a child with an NTD. This higher dose is available only on prescription from your GP. Your GP or midwife may also recommend additional screening tests during your pregnancy.
There are also a number of conditions that increase the risk of folic acid deficiency (not having enough in your body).
Anyone taking anticonvulsant medicines, undergoing kidney dialysis, or with conditions such as liver disease, may need to supplement their diet with additional folic acid. This will also apply if you have malabsorption (difficulty absorbing vitamins and minerals) or certain types of anaemia.
If you are taking medication for a medical condition, check with your GP before taking folic acid supplements.

(from the NHS Direct website)

Folic acid is also important for many other body processes, including the growth and reproduction of all cells.
Folic acid along with vitamin B12 and vitamin B6 also decreases blood levels of homocysteine, high levels of which can damage blood vessel walls and lead to various circulatory problems such as heart disease and strokes.
Folic acid may interact with certain antibacterial drugs, for example sulphonamides. The usual advice applies here; if you are on medication and want to take folic acid ask your doctor first.