Physiological Changes During Pregnancy

Cardiovascular Adaptations

The American College of Obstetricians and Gynecologists notes that uteroplacental blood flow increases from 50 mL/min to close to 1000 mL/min during pregnancy, receiving up to 20% of maternal cardiac output at term, contributing to the hyperdynamic circulatory state 1
Inferior vena caval compression by the gravid uterus in the supine position causes an abrupt decrease in cardiac preload, leading to hypotension with weakness and lightheadedness—this explains the patient's positional dyspnea 2
The left lateral position significantly increases cardiac output compared to the supine position, which is why pregnant women require multiple pillows for sleep 2
Pregnancy is associated with increased circulating plasma volume (up to 50%), creating a hyperdynamic state similar to decompensated cirrhosis 5, 1

The American Thoracic Society notes that minute ventilation increases 20-40% above baseline by term, driven by progesterone-mediated increased respiratory center sensitivity, creating the sensation of dyspnea despite adequate oxygenation 1, 3
Oxygen consumption rises 20-33% above baseline by the third trimester due to increased metabolic demands of the fetus, placenta, and maternal organs 3
The combination of reduced functional residual capacity (down 10-25%) and increased oxygen consumption creates the paradox of dyspnea with normal oxygen saturation 3

The American Gastroenterological Association notes that heartburn results from progesterone-induced relaxation of the lower esophageal sphincter, facilitating gastroesophageal reflux, affecting up to 80% of women in the third trimester 4
Progesterone has an inhibitory effect on GI and small bowel motility, leading to delayed gastric emptying and worsening reflux symptoms 4
The elevated position of the stomach due to diaphragmatic elevation by the gravid uterus mechanically worsens reflux 5, 4

The American College of Obstetricians and Gynecologists recommends that for dyspnea, reassurance that symptoms are physiological; recommend left lateral positioning to optimize cardiac output and relieve vena caval compression 1, 2
Left lateral positioning optimizes cardiac output by relieving vena caval compression and should be recommended for sleep and rest 1, 2
Patients should maintain moderate-vigorous physical activity and not restrict exercise based on mild dyspnea alone 3