If you have a headache when you're pregnant, what painkiller should you take? Much of what you take into your body will reach your developing fetus, so do you avoid drugs completely or are some medicines less risky than others?

The solution

The British National Formulary – used by doctors to look up the doses and safety of drugs – warns that the main risk of drugs producing congenital malformations in the fetus falls between the third and the 11th week of pregnancy. But in the second and third trimester drugs can still affect development and damage tissue. For example, isotretinoin, a drug used in the treatment of acne, should be totally avoided because it can damage the fetus's brain throughout pregnancy. The UK Teratology Information Service (UKTIS) states that half of babies born to women taking isotretinon may develop learning difficulties.

The BNF says that drugs should only be prescribed when the benefit to the mother is greater than the risk to the fetus. Thalidomide, a drug given to pregnant women to relieve morning sickness in the 1950s, is thought to have killed 40% of babies whose mothers took it and caused severe deformities in others. The risks of all medicines (including those available over the counter) in pregnancy has been at the forefront of mothers' and doctors' minds ever since.

If you need to take any medication, talk to your doctor first. In the US, the FDA has a helpful list that classifies the safety of drugs in pregnancy from A (studies show no risk to the fetus) to X (evidence of risk to the fetus).

If you are taking drugs for a medical condition such as asthma, diabetes, high blood pressure or depression consult with your GP as these conditions are likely to need ongoing treatment. Salbutamol asthma inhalers, for instance, are considered safe to use.

It is also worth remembering that sometimes medication can reduce risks to the fetus. For example if you have a urinary tract infection, your doctor will usually prescribe antibiotics. If an infection takes hold and you develop a high temperature, the risk to the fetus is much higher than that of taking a tried and tested medicine.

If you get a headaches, paracetamol is also considered safe for short periods – however, be careful not to exceed the recommended dose. Ibuprofen is best avoided as it may increase miscarriage risk in the first trimester of pregnancy and in the third trimester can affect the fetus's lungs, reduce amniotic fluid levels or delay labour. For most common problems it is better to try non-medical remedies first. Constipation, for example, should be relieved by drinking more fluids and eating plenty of fruit and vegetables.