How Medicines and Other Exposures Can Affect Pregnancy

Pregnancy can change the benefits and risks of medicines and other exposures. Some substances cross the placenta, but crossing does not automatically mean harm. Safe decisions compare the risk of an exposure with the risk of leaving an illness untreated.

Exposure does not mean harm An exposure is contact with a medicine, chemical, infection, radiation source or substance during pregnancy. The word does not describe the outcome. Many people take a medicine or encounter an exposure before realising they are pregnant. Most individual exposures do not automatically cause miscarriage or a congenital condition. Risk assessment begins by identifying exactly what occurred rather than assuming the worst. Every pregnancy has a background risk Miscarriage and congenital conditions occur in pregnancies without any identified harmful exposure. This is called the background risk. When researchers assess a medicine, they compare outcomes among exposed pregnancies with the expected background pattern. A reported problem after a medicine was taken does not prove that the medicine caused it. The underlying illness, other medicines and chance may also contribute. The placenta is not an absolute barrier Many medicines and chemicals can cross the placenta. Transfer depends on molecular size, fat solubility, protein binding and concentration. Placental transfer does not make a medicine unsafe by itself. Some medicines cross without causing recognised harm. Others create risk only at particular doses or stages. Some medicines mainly affect the pregnant person while indirectly affecting the pregnancy through blood pressure, breathing or metabolism. Timing matters Embryonic and fetal tissues develop at different times. Very early exposure may have a different effect from exposure during organ formation or later growth. The first trimester includes the main period when major organs are forming. Certain teratogenic exposures during this period can increase the chance of structural differences. Later exposures may affect growth, organ function, blood flow or newborn adaptation. These are broad principles, not a calendar that predicts one pregnancy outcome. Gestational age, exact timing and developmental biology must be considered together. Dose and duration matter A single low exposure may create a different risk from repeated or high dose exposure. The route also matters. A small amount applied to intact skin may produce less bloodstream exposure than a swallowed medicine. Some medicines accumulate in the body. Others leave the circulation quickly. Risk assessment should therefore include the exact product, strength, amount, route and dates. Maternal illness also matters Stopping treatment can be more harmful than continuing it. Poorly controlled epilepsy can cause seizures, injury and reduced oxygen. Severe asthma can impair maternal and fetal oxygenation. Uncontrolled diabetes, hypertension, infection or mental illness can also harm both people. The question is therefore not simply whether a medicine has any risk. The relevant comparison is between the best available treatment and the consequences of untreated or undertreated illness. Do not stop prescribed medicines suddenly A person who becomes pregnant while taking regular medicines should seek prompt advice. They should not usually stop treatment abruptly unless an appropriate clinician advises this. Sudden withdrawal can cause relapse, seizures, adrenal crisis, uncontrolled blood pressure or other harm. A prescriber may continue the medicine, adjust the dose, change treatment or arrange additional monitoring. The safest decision depends on the medicine and the condition being treated. Preconception review A medicine review is ideally completed before pregnancy when possible. The clinician checks whether each medicine is still required, whether a safer effective alternative exists, whether the dose is appropriate. Whether folic acid or additional monitoring is needed, whether the medicine requires a pregnancy prevention programme, and what might happen if treatment is changed. A review should include prescribed medicines, pharmacy purchases, supplements, creams, injections and herbal products. Pregnancy planning must not be used to deny essential treatment. Teratogens A teratogen is an exposure that can increase the chance of altered embryonic or fetal development. Teratogenicity is not an all or nothing property. Risk may depend on timing, dose and the susceptibility of the pregnancy. Examples of medicines with important pregnancy risks include valproate, isotretinoin, methotrexate and some anticoagulants. The exact risk and required action differ for each medicine. A list should never replace an individual review. Valproate and antiseizure medicines Valproate exposure during pregnancy is associated with major risks to fetal development. Strict regulatory controls therefore apply to its use in people who could become pregnant. However, stopping valproate suddenly can cause dangerous seizures. Anyone who becomes pregnant while taking valproate needs urgent specialist advice but should not stop it without medical guidance. Other antiseizure medicines have different risk profiles. The aim is effective seizure control with the safest appropriate treatment. Retinoids Oral retinoids such as isotretinoin can cause severe fetal harm. Pregnancy prevention requirements apply before, during and after treatment for specified periods. Topical retinoids produce much lower systemic exposure but are generally avoided during pregnancy as a precaution. Someone who becomes pregnant during oral retinoid treatment should stop taking further doses and contact the prescribing service urgently. This medicine specific action does not create a general rule to stop every prescribed medicine. Blood pressure medicines Some medicines affecting the renin angiotensin system can harm fetal kidneys and amniotic fluid production, particularly later in pregnancy. People taking these medicines should receive prompt review when planning pregnancy or after a positive test. Alternative treatments are available for many patients. Stopping treatment without replacement may leave severe hypertension uncontrolled. Anticoagulants Pregnancy changes blood clot risk, so some people require anticoagulation. Warfarin can cross the placenta and has important fetal risks. Low molecular weight heparin does not cross the placenta and is used in many pregnancy situations. The correct medicine depends on the indication, gestation and specialist assessment. A person should never substitute one anticoagulant for another independently. Pain relief Paracetamol is commonly used during pregnancy when clinically appropriate. It should be taken at the recommended dose and for the shortest necessary period. Non steroidal anti inflammatory drugs, such as ibuprofen, require caution and are unsuitable at some stages. Persistent pain deserves assessment because repeated self treatment can conceal an underlying problem. "Natural" pain remedies can also contain pharmacologically active substances. Medicines bought without prescription Pharmacy medicines are not automatically safe for every pregnancy. Cold remedies may contain decongestants, antihistamines, cough suppressants or multiple active ingredients. Some indigestion preparations alter absorption of other medicines. Vitamin supplements can contain excessive vitamin A or other ingredients unsuitable during pregnancy. A pharmacist can help identify the active ingredients and check compatibility. Herbal and complementary products Herbal products can have biological effects. Their concentration, purity and interactions may be uncertain. Traditional use does not prove safety during pregnancy. Some products may affect the uterus, liver, blood pressure or clotting. A clinician needs the exact product name and ingredient list. Unregulated products should not be described as safer than licensed medicines. Alcohol Alcohol crosses the placenta. The fetus cannot process alcohol in the same way as an adult. Higher and repeated exposure increases the risk of fetal alcohol spectrum disorder and other adverse outcomes. Current UK guidance states that the safest approach during pregnancy or when planning pregnancy is not to drink alcohol. A person who drank a small amount before recognising the pregnancy should not be shamed. They should stop further exposure and discuss concerns honestly with a midwife or clinician. Smoking and nicotine Smoking exposes the pregnancy to nicotine, carbon monoxide and many other chemicals. Carbon monoxide reduces the oxygen carrying capacity of maternal blood. Smoking is associated with placental complications, fetal growth problems, preterm birth and other harms. Stopping at any stage provides benefit. Support can include behavioural help and appropriately supervised nicotine replacement treatment. Vaping avoids combustion products found in cigarettes but is not free from uncertainty or nicotine exposure. Someone unable to stop immediately should receive practical support rather than blame. Cannabis and other recreational drugs Cannabis and other drugs can affect maternal consciousness, cardiovascular function, nutrition and placental blood flow. Some substances can also cause fetal or newborn effects. Street products may contain unknown doses or additional substances. Sudden withdrawal from alcohol, benzodiazepines or opioids can be medically dangerous. People using substances regularly should seek specialist support rather than stopping without a safe plan. Disclosure should lead to healthcare and safeguarding support, not punishment. Caffeine Caffeine crosses the placenta and is cleared slowly by the fetus. High intake is associated with adverse pregnancy outcomes. Caffeine is found in coffee, tea, cola, energy drinks, chocolate and some medicines. UK advice generally recommends limiting caffeine to no more than 200 milligrams daily during pregnancy. Actual content varies considerably between products. Vitamins and supplements Folic acid reduces the risk of neural tube defects. Most people are advised to take 400 micrograms daily before pregnancy and through the first 12 weeks. A higher prescribed dose is recommended for some risk groups. Vitamin D supplementation is also advised according to UK guidance. Supplements containing vitamin A in the form of retinol should be avoided unless specifically prescribed. More is not automatically better. Food and infection exposures Some foods carry increased risk from listeria, toxoplasma, mercury or other contaminants. Food advice aims to reduce avoidable risk while maintaining adequate nutrition. Cat faeces and undercooked meat can transmit toxoplasma. Unpasteurised or contaminated foods can transmit listeria. Travel, occupation and contact with infections may also matter. An exposure does not prove infection. Testing depends on the organism, timing and symptoms. Vaccines Some vaccines are recommended during pregnancy because infection creates substantial risk. Current UK pregnancy programmes include vaccination against influenza, whooping cough (pertussis) and RSV. RSV vaccination is recommended during every pregnancy from 28 weeks. Recommendations can change, so current NHS advice should always be checked. Non live vaccines generally have an established role when indicated. Live vaccines are usually avoided during pregnancy. Accidental receipt of a live vaccine does not automatically mean fetal harm or that a pregnancy should end. Specialist assessment considers the specific vaccine and evidence. Diagnostic imaging Ultrasound and MRI do not use ionising radiation. X rays, CT scans and nuclear medicine tests do use ionising radiation. The fetal dose from most diagnostic examinations is far below levels associated with deterministic fetal injury. A medically necessary investigation should not be withheld merely because someone is pregnant. The imaging team adjusts the examination, considers alternatives and limits exposure appropriately. Delaying diagnosis of serious illness can be more dangerous than the radiation from an indicated test. Workplace and environmental exposures Potential concerns include solvents, pesticides, anaesthetic gases, heavy metals, radiation and infectious agents. Risk depends on the substance and actual exposure rather than the job title alone. Employers have duties to assess workplace risks during pregnancy. Occupational health advice can identify whether changes in ventilation, protective equipment or duties are required. Unnecessary exclusion from work can create financial and psychological harm. What to do after an accidental exposure First, identify the exact substance. Record the product or medicine name, active ingredients, dose or amount, route, date and time, frequency, gestational age, symptoms, and other medicines or exposures. Contact a midwife, GP, pharmacist, specialist service or NHS 111 according to urgency. UK Teratology Information Service resources provide evidence based medicine information. A risk assessment may result in reassurance, a medication change, testing or additional fetal monitoring. It should not begin with blame. Internet information can mislead Package leaflets may contain precautionary wording because pregnancy studies are limited. Online reports may describe isolated events without proving causation. Conversely, absence of a warning does not prove safety. Reliable assessment uses regulated product information, current guidance, teratology services and the clinical context.

The effect of a pregnancy exposure depends on the substance, dose, timing and clinical context, and stopping necessary treatment can also cause harm.