Pregnancy places a high metabolic demand on the maternal body, requiring increased oxygen delivery to support the growing fetus, placenta, and enlarged maternal organs. To meet this need, the respiratory system undergoes profound functional changes driven primarily by circulating hormones. Progesterone acts as a key respiratory stimulant by directly acting on the respiratory center in the medulla oblongata. This hormonal stimulation increases the woman's sensitivity to carbon dioxide, encouraging deeper and more frequent breathing. As a result, minute ventilation rises significantly early in gestation, often leading pregnant women to feel short of breath even when their actual oxygen levels are completely normal.
As the uterus enlarges to accommodate the developing fetus, it pushes upward against the abdominal contents and exerts pressure on the diaphragm. By the third trimester, the diaphragm is elevated by approximately 4 centimeters. Despite this physical crowding, the thoracic cavity adapts by expanding in other directions. The subcostal angle widens, the lower ribs flare outward, and the transverse diameter of the chest increases by about 2 centimeters. These structural shifts ensure that chest wall movement remains effective despite the upward displacement of the diaphragm.
Pregnancy alters several pulmonary volumes while leaving others surprisingly stable.
The hyperventilation induced by progesterone causes women to exhale a greater volume of carbon dioxide than usual. This leads to a mild state of chronic respiratory alkalosis. To compensate for this lowered blood carbon dioxide level, the kidneys increase the excretion of bicarbonate in the urine, maintaining the blood pH within a normal, slightly alkaline range. This subtle shift creates a favorable concentration gradient for carbon dioxide to diffuse easily across the placenta from the fetal circulation into the maternal blood.
Up to 70 percent of healthy pregnant women experience physiological breathlessness, often described as a subjective awareness of needing more air. This is a normal adaptation driven by increased tidal volume and lower carbon dioxide thresholds, not a sign of pathology. However, midwives must be vigilant in distinguishing this benign dyspnea from warning signs of serious conditions like pulmonary embolism, asthma exacerbation, or cardiac compromise. Red flags requiring immediate medical investigation include sudden chest pain, persistent coughing, hemoptysis, or any dyspnea accompanied by cyanosis and tachycardia.