Table 3.

Physiological changes of respiratory system during the pregnancy [37, 39–45].

Feature	Change	Week of pregnancy/trimester
Chest compliance	↓
Total lung capacity	↓ By 5%
Vital capacity	=
The functional residual capacity	↓ By 10-25%
Functional residual capacity	↓ By 10-25%
Expiratory reserve volume	↓ By 15-20%
Tidal volume	↑ By 30-50% (from 500 to 700 mL)
Minute volume	↑ By 40%	During 1st trimester
Respiratory rate	↑ By 1-2 breaths more than normal
Minute ventilation	↑ By 20–50% (from 7.5 to 10.5 L/min)
Residual volume	↓ By 20-25%
The expiratory reserve volume	↓ By 15–20% (200-300 mL)
The residual volume	↓ by 20–25% (200-400 mL)
Total lung capacity	↓
Respiratory capacity	↑ By 5-10%
Inspiratory capacity	↑ By 5–10% (200–350 mL)
The total lung capacity	↓ By minimally 5%	By term
Spirometry parameters (FVC, FEV1)	=
Oxygen consumption	↑ By 20-40%
Metabolic rate	↑ By 15%
Oxygen reservoir	↓
PaO2	↑
PaCO2	↓
Bicarbonate levels	↓ To 18–21 mmol/L
Blood pH	↑ To 7.44 (respiratory alkalosis with a compensatory metabolic acidosis)

Abbreviations: FVC: forced vital capacity; FEV1: forced expiratory volume in 1 second; PaO₂: partial pressure of oxygen in the arterial blood; PaCO₂: partial pressure of carbon dioxide in the arterial blood.