Maternal Medicine - Asthma

Normal respiratory physiology during pregnancy

Increased oxygen demand causes increased tidal volume and reduced PaCO2. A compensated respiratory alkalosis is normal, and women are often aware of the increased ventilation as subjective breathlessness.

Course of asthma during pregnancy

  • About a third each improve, deteriorate, or are unchanged.
  • Severe pre-pregnancy asthma is more likely to deteriorate during pregnancy.
  • Post-natal deterioration is common, perhaps due to loss of endogenous steroids.
  • Asthma per se usually has little effect on pregnancy, though poorly-controlled asthma is associated with some increases in frequency of adverse outcomes.


  • Treatment is basically the same as in non-pregnant women. Most treatments not thought to be harmful. Many women reduce or stop treatment on learning of pregnancy - this should be discouraged, as poorly-controlled asthma may be harmful.
  • Prednisolone is deactivated by the placenta - only 10% crosses over.
  • Long-term steroids increase the risk of premature rupture of membranes and gestational diabetes.
  • Theophylline/aminophylline do cross the placenta - no known harm, but use less in pregnancy.
  • Leucotriene antagonists have little data - generally continue if already established therapy, but avoid starting.
  • Aspirin is increasingly used for prophylaxis of pre-eclampsia - remember to check sensitivity!
  • CXR should be performed as normally indicated in severe asthma (fetal radiation exposure is 1/20th max. recommended dose).
  • Steroids are OK in breast-feeding women if prednisolone<30mg/day.