There are some important principles that should be applied when providing advice on the use of medicines in pregnancy.

The importance of applying the principles

Applying good principles of prescribing is important when prescribing for a woman of childbearing age or for a man trying to father a child.

Clear principles of prescribing are an important means of addressing the challenges of prescribing in pregnancy. These challenges are related to the medicines and to the pregnancy. They include:


Over 80 per cent of women in the UK report taking medicines during their pregnancy.

Medicines and chemicals are thought to account for approximately 4-6% of birth defects.

Medicines can have harmful effects on the embryo or fetus (teratogenic) at any time during pregnancy.

Information is lacking on potential adverse fetal effects of medicines and pregnancy outcomes, particularly newer medicines.


Physiological changes that occur during pregnancy can affect the pharmacokinetics of medicines.

A teratogen may directly or indirectly cause structural or functional abnormalities in the fetus or in the child after birth, some of which may not be apparent until later life.

Key principles


Therapeutic benefit

  • Only consider using a medicine when it is really needed.
  • Only use a medicine when a risk v benefit assessment has been completed and the expected benefits (usually to the mother) outweigh the risks (usually to the fetus).
  • Always do a risk v benefit assessment on an individual patient basis – ensure the most up-to-date information is used.


  • Use the lowest effective dose for the shortest possible time  – careful monitoring is important to ensure that the doses used are as low as possible but provide an adequate therapeutic response.
  • Avoid new medicines wherever possible as there is a lack of information on their use in pregnancy – absence of information does not imply safety.
  • Avoid poly-pharmacy – teratogenicity of a medicine may be enhanced by co-administration of a second medicine or more. Risk factors may not only be additive but also potentially synergistic.
  • Avoid alcohol, tobacco and other recreational substances.

Stage of pregnancy

  • The stage of exposure is critical in determining if the embryo is at a susceptible stage of development.
  • The type of birth defect that results from exposure depends on the particular stage of development when the exposure occurs.
  • Individual medicines can have variable periods of sensitivity:

First trimester

  • Avoid using medicines in the first trimester wherever possible – the period of greatest susceptibility to teratogenic effects (i.e. malformations) is usually the first 12 weeks of pregnancy.

The main risk is for structural defects as the major structures (such as brain, spinal chord, arms and legs) and organs are developing.

Second and Third trimester

  • Exposure to some medicines poses a risk later in pregnancy, when exposure may affect fetal growth or functional development. Later exposure can also have adverse or toxic effects on fetal tissues.

Medicines given shortly before term or during labour, can have adverse effects on labour or on the neonate after delivery e.g. withdrawal effects.

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