Expansion
Gas exchange, nutrition, excretion, endocrine and barrier functions
Expansion
Functions
- Gas exchange: by diffusion, aided by fetal haemoglobin and the double Bohr effect
- Nutrition: glucose by facilitated diffusion, amino acids by active transport, fatty acids by diffusion
- Excretion of fetal waste
- Endocrine: hCG, progesterone, oestrogen, human placental lactogen
- Immunological: transfers maternal IgG, giving passive immunity, while avoiding rejection of the fetal allograft
Transfer across the placenta is favoured by low molecular weight, lipid solubility, being un-ionised, and low protein binding.
- Cross readily: most drugs, alcohol, nicotine, warfarin, opioids, anaesthetic agents, IgG, and viruses such as rubella and cytomegalovirus
- Do not cross appreciably: heparin (large), insulin (large), glycopyrronium (ionised), IgM, and most large protein hormones
This explains two familiar clinical rules: heparin is the anticoagulant of choice in pregnancy while warfarin is teratogenic, and neonatal IgM indicates congenital infection because maternal IgM cannot cross.
Uteroplacental blood flow rises to about 700 ml/min at term and is not autoregulated, so it falls directly with maternal hypotension.