Placenta

• Most drugs cross the placenta - except large molecular weight e.g. heparin
• Small, lipid soluble drugs cross more quickly

Pharmacokinetics

• Absorption may be affected by morning sickness
• Increased plasma volume and fat stores - volume of distribution increases
• Decreased protein binding - increased free drug
• Increased liver metabolism of some drugs e.g. phenytoin
• Elimination of renally excreted drugs increases - increased GFR
• May need to check concentrations and alter dose during pregnancy and after delivery e.g. lithium or digoxin

Pharmacodynamics

• No significant changes
• Pregnant may be more sensitive to some drugs e.g. hypotension with antihypertensives in second trimester

Prescribing considerations in the different stages of pregnancy

Pre-conception

• Folic acid 400mcg daily for 3 months prior and 3 months of pregnancy
• Counselling RE chronic conditions - optimise therapy to choose safest drugs, review whether drug therapy necessary

1st trimester

• Avoid all drugs if at all possible unless maternal benefit outweighs risk to foetus
  • Risk of early miscarriage
  • Organogenesis
  • Period of greatest terotogenic risk - 4th-11th week

2nd and 3rd trimesters