Maternal informed choice in the administration of analgesics is essential. 2. The placenta is not a barrier to the transfer of drugs to the fetus. 3. All analgesic and anaesthetic drugs are lipid soluble and will cross the placenta. 4. Fetal acidosis increases the accumulation of some drugs in the fetal compartment. 5. Reduced maternal blood flow to the placenta reduces the placental transfer of some drugs.