All drugs cross the placenta to some extent.


General Advice

  • Consider non-drug measures first; avoid all drug therapy if possible, especially in the first trimester.
  • Use particular care when prescribing for women of childbearing potential. Half of all pregnancies are unplanned.
  • Poor disease control (e.g. asthma, epilepsy) may carry a higher foetal risk than drug treatment.
  • If drug treatment is necessary, use drugs with a more established safety record.
  • Use the lowest effective dose for the shortest possible time.
  • Readily seek advice because:
    • Readily available references usually offer little in terms of risk assessment.
    • Grading systems are over-simplified. Clinical context is important.


Effect of Drugs on Pregnancy, Foetus, or Neonate


  • Approximately 2 to 4% of live births are associated with a foetal abnormality. Of these, drugs are responsible for 1 to 5% (i.e. affecting < 0.2% of all live births).
  • If pregnancy is planned or known, drug-associated malformations may be preventable.
  • Accurate timing of drug exposure and examination of the drug pharmacokinetics may help determine the risk.


Drugs with proven teratogenicity in humans

Note: This list is not exhaustive.

ACE inhibitors Cytotoxic agents Tetracyclines (e.g. Doxycycline)
Alcohol Leflunomide Thalidomide
Amiodarone Lithium Trimethoprim (1st trimester)
Androgens (not accidental oral contraceptive use) Methotrexate Warfarin
Anticonvulsants (especially valproate sodium) Misoprostol
Carbimazole Retinoids (e.g. Isotretinoin)


Predictable Risks Based on Drug Activity

  • NSAIDs cause premature closure of the ductus arteriosus in the third trimester.
  • Some antihypertensives may reduce placental perfusion, potentially causing foetal hypoxia and in utero growth retardation.
  • Withdrawal reactions or excessive clinical effects may be observed in neonates after in utero exposure to some drugs (e.g. respiratory depression in neonate from maternal use of opioids).
  • Beta-agonists and NSAIDs may delay delivery.
  • Some drugs are abortifacient (e.g. misoprostol).


Effects of Pregnancy on Drug Pharmacokinetics

  • Most drugs undergo increased clearance during pregnancy due to increased organ blood flow and enzyme induction.
  • Therapeutic Drug Monitoring (TDM) is complicated during pregnancy (e.g. lamotrigine). Seek specialist advice (e.g. Medicines Information)
  • Protein binding changes in pregnancy and this may affect the interpretation of some drug concentrations (e.g. phenytoin).