Prescribing in pregnancy requires balancing maternal and fetal risks. This guidance outlines key risks and principles to ensure safe, informed decisions.

Consider the risks

Before prescribing medicines during pregnancy, you should consider the risks to both the mother and fetus. These include:

Background risk

All pregnancies have some risk. There is a background risk of 2 to 3% of birth defects and 10 to 20% of miscarriage, irrespective of any medicines used during pregnancy. 

Risk from medicines

Over 80% of women in the UK report taking medicines during their pregnancy. Medicines and chemicals are thought to account for approximately 2.5% of all birth defects.

Some medicines are known or suspected to increase the risk of birth defects and developmental disorders due to their teratogenic potential. Medicines can have harmful effects on the embryo or fetus (be teratogenic) at any time during pregnancy.

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

The benefit of treating the mother and maintaining good maternal health must be considered. Treating the mother is also an important factor in supporting good fetal health and development.

Antenatal risk

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.

Maternal risk

The risk of miscarriage, having a baby with a birth defect or complications during pregnancy may be increased by factors other than medicines including (but not limited to):

Increasing age

Pregnant women over age 35 have a higher risk of fetal chromosomal abnormalities and miscarriage.

Maternal health

Pre-existing conditions such as high blood pressure and diabetes have a higher risk of miscarriage or other complications.

Conditions that occur during pregnancy like preeclampsia or gestational diabetes can be a risk for the pregnant woman or fetus if not treated.

Drinking alcohol, smoking or using recreational drugs during pregnancy can cause harm to fetal development.

Poor obstetric history

Poor outcomes or congenital malformations in previous pregnancies can sometimes mean there is a higher risk of these occurring in subsequent pregnancies.

Paternal risk

Paternal factors can influence the outcome of a pregnancy. These include (but are not limited to):

Increasing age

An increase in miscarriage rate is associated with pregnancies where the father is over the age of 45. The effect of paternal age is less pronounced than that observed with advanced maternal age.

Paternal health

Exposures to certain substances or environmental factors before or during conception can affect sperm quality and fertility. This includes exposure to medicines, chemicals, tobacco, alcohol and other substances.

While most paternal exposures do not directly cause birth defects, they can impact sperm and potentially affect pregnancy outcomes. Advice can vary depending on the medicine and may involve suggestions such as delaying conception for around six months (two sperm cycles) if the man has been exposed to cytotoxic or mutagenic agents.

Apply key principles

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

The principles help address the challenges and risks associated with prescribing during pregnancy. The principles are:

Prescribe only when beneficial

Use medicines only when clearly indicated.

Always do a risk versus benefit assessment on an individual patient basis using the most up-to-date information.

Only use a medicine when the expected benefits (usually to the mother) outweigh the risks (usually to the fetus).

Consider non-pharmacological options as an alternative where appropriate.

Use the minimal effective treatment

Prescribe the lowest effective dose for the shortest possible time. Carefully monitor to confirm an adequate therapeutic response.

Avoid known teratogens

These can disrupt the development of a fetus leading to birth defects, developmental delays, miscarriage, stillbirth and growth restrictions.

Avoid newly licensed medicines wherever possible

Select medicines with proven pregnancy safety records. Avoid new or untried medicines where alternatives exist. Lack of data doesn’t imply safety.

Avoid polypharmacy

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 using medicines in the first trimester where possible

The first 12 weeks of pregnancy pose the highest susceptibility to teratogenic effects, such as malformations.

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

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, such as withdrawal effects.

Update history

  1. Broken hyperlink fixed
  1. Full review and update. Title changed from 'The principles of prescribing in pregnancy' to ' The risks and principles of prescribing in pregnancy'.
  2. Published

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