You have good reason to be concerned as research shows that drinking even small amounts of alcohol during pregnancy may affect the unborn child.

When a woman who's pregnant drinks alcohol, the levels of alcohol in her baby's blood rise as high as her own. But because the baby's liver is immature, it can't metabolise (or break down) that alcohol as fast as an adult can. So the baby is exposed to greater amounts of alcohol for longer than the mother.

Foetal alcohol syndrome

When an unborn baby is constantly exposed to alcohol, a particular group of problems can develop, known as foetal alcohol syndrome. This consists of:

• Growth problems, starting with low birth weight
• Problems with the central nervous system such as developmental delays and learning difficulties, restlessness and irritability
• Abnormal facial features, including small head, narrow eye slits, a flat face and loss of the groove between nose and upper lip
• Other abnormal body parts include muscles, heart and kidneys

All these problems are permanent, although surgery can correct some features.

How much is too much?

Pregnant women should avoid alcohol completely, the government advises, although the Royal College of Obstetricians and Gynaecologists says there's no evidence a couple of units once or twice a week will harm the baby. Binges (for example, getting drunk) are definitely to be avoided.

The main problem is that the levels of alcohol and its effect vary from person to person so you can't predict the harm. Some women drink heavily with no apparent problem, while some babies are harmed when their mothers drink only moderate amounts.

The greatest risk is probably in the early stages of the pregnancy, when rapid growth by the baby may be disrupted and abnormal features may develop.

Later in pregnancy, drinking alcohol increases the risk of miscarriage, and impairs overall growth. Alcohol is particularly toxic to brain cells at this stage, resulting in a small brain.

Be firm but supportive

Scaring your sister may just drive her away from you, at a time when she really needs you. Try to be supportive and understanding while guiding her towards more expert help.

Although the father's drinking habits don't directly cause foetal alcohol syndrome, it can strongly influence a woman's own behaviour. Ensure your sister's partner is also aware of your concerns.

This article was last medically reviewed by Dr Trisha Macnair in June 2008