Increased estrogen levels during pregnancy causes mucosal edema, hyperemia, mucus hypersecretion, capillary congestion, and increased fragility in the upper air ways, most markedly during the third trimester of pregnancy.

Gravid uterus causes changes in the configuration of the thorax during pregnancy which include an increase in the circumference of the lower chest wall, elevation of the diaphragm (displacement of approximately 4 cm to 5 cm) and a 50% widening of the costal angle. These changes peak around the 37th week of pregnancy.

Progesterone acts as trigger of the primary respiratory centre by increasing the sensitivity of the respiratory centre to carbon dioxide.

All these physiological changes play a great role in exacerbations of asthma during pregnancy.

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