A drug is a teratogen if its administration to the pregnant mother, directly or indirectly, causes a structural or functional change in the fetus or child.
Organ formation within the fetus occurs in the first 3 months of pregnancy, teratogenic drugs taken in this period therefore tend to cause structural defects.
From 3 months onwards, teratogenic drugs tend to cause growth defects.
Women need to be aware of medications they are taking if they are planning a pregnancy, so that if any of the medications are potentially teratogenic these can be either stopped, switched or reduced to the lowest dose possible.
The risk to the fetus must always be balanced with the risk to the woman if a medication is stopped.
Resource for medications in pregnancy
The primary clinical toxicology database of the National Poisons Information Service
TOXBASECommon medications to be aware of:
Drugs used in Hypertension
Angiotensin-Converting Enzyme (ACE) Inhibitors are relatively safe in the 1st trimester, but can cause significant problems such as renal tubular aplasia and Intra Uterine Growth Retardation in the fetus if taken in the 2nd and 3rd trimesters. Medication changes should be discussed with specialists within medical-obstetric services. Statins are teratogenic and ideally should be stopped at least 3 months prior to conception.
Antiepilepsy Drugs (AED)
Most AEDs are teratogenic, although the risk is reduced with monotherapy. Some AEDs are potentially less likely to cause problems, but the risk to the fetus needs to be balanced with the risk of seizures in the mother which puts both the mother and the baby at possible harm.
See Epilepsy in these pages and the British National Formulary:
Alcohol
Fetal Alcohol Syndrome is seen in babies born to mothers who drink excessive amounts of alcohol and no safe threshold amount can be defined.
Herbal medications
There is little information on safety during pregnancy so these should be avoided.
Antibiotics
Penicillins, erythromycin and cephalosporins are safe. Antibiotics to be avoided are streptomycin, the tetracycline family in the 3rd trimester and trimethoprim which interferes with folate metabolism and so should be avoided in the 1st trimester.
Vitamin A
Preparations such as retinoic acid can cause malformation of the ear.
Warfarin
Should be avoided in the 1st trimester as it is linked to a number of developmental abnormalities and also avoided in the lead up to delivery.
Lithium
Where possible lithium should be avoided in pregnancy, especially in the 1st trimester, as it can sometimes cause an abnormality of the heart valves called Ebstein’s anomaly. All women on lithium should have a high resolution USS and fetal echocardiography at 18-20 weeks of pregnancy.
