Is mild left atrial dilatation (left atrial enlargement) a normal finding in a pregnant woman in her third trimester with no prior history of cardiovascular disease?

Mild Left Atrial Dilatation in Late Pregnancy

Yes, mild left atrial dilatation is a normal physiological finding in late pregnancy and does not indicate pathology in women without pre-existing cardiovascular disease. 1

Physiological Basis for Left Atrial Enlargement

The heart can increase its size by up to 30% during pregnancy, which is partially due to dilatation of cardiac chambers. 1 This adaptation occurs to accommodate the profound hemodynamic changes of pregnancy:

• Plasma volume increases by 40% above baseline by 24 weeks gestation, creating a chronic volume-overload state 1, 2
• Cardiac output increases 30-50% above baseline, reaching maximum at 32 weeks gestation 1, 2
• Stroke volume rises significantly, particularly in early pregnancy, contributing to increased cardiac output 1

Specific Evidence for Left Atrial Changes

Left atrial area measured by two-dimensional echocardiography is significantly larger during the third trimester (16.7 ± 4.0 cm²) compared with postpartum measurements (13.8 ± 3.1 cm²) and non-pregnant controls (15.5 ± 3.5 cm²). 3 This represents a well-documented normal finding.

Using more sensitive three-dimensional echocardiography, researchers have demonstrated:

• Left atrial end-systolic volume index increases significantly from 19.4 to 24.7 mm² during pregnancy 4
• Left atrial stroke volume index increases from 12.0 to 15.1 mL/m² as pregnancy progresses 4
• These changes are completely reversible postpartum, confirming their physiological nature 5, 3

Functional Adaptations Accompanying Atrial Enlargement

The enlarged left atrium maintains normal function through compensatory mechanisms:

• Left atrial emptying fraction remains unchanged despite increased volume 4
• Reservoir function increases during pregnancy to accommodate higher venous return 5
• Booster pump function increases with enhanced atrial contraction (A-wave) to maintain cardiac output 4, 5
• Conduit function decreases as reflected by reduced E/A ratio, but this is a normal adaptation 5, 3, 6

Clinical Implications and Pitfalls

The key distinction is that mild left atrial dilatation with preserved function and normal blood pressure represents physiological adaptation, not pathology. 4 However, clinicians must avoid several common pitfalls:

• Do not confuse physiological left atrial enlargement with pathological conditions such as mitral stenosis, which causes sharp rises in left atrial pressure and precipitates pulmonary edema 2
• Echocardiography should be performed if there are unexplained or new cardiovascular signs or symptoms, not simply because mild left atrial dilatation is noted 1
• Normal pregnancy is accompanied by Doppler evidence of "physiological" mitral regurgitation in the absence of structural valve disease, which should not be misinterpreted as pathology 7

When Left Atrial Dilatation Becomes Concerning

Left atrial enlargement warrants further investigation when accompanied by:

• Symptoms such as paroxysmal nocturnal dyspnea, anginal chest pain, or syncope 8
• Evidence of elevated left atrial pressure with pulmonary edema 2
• Structural valve abnormalities on echocardiography 7
• Significantly elevated pulmonary artery pressures (>40 mmHg suggests pathology rather than physiological adaptation) 7, 4
• Reduced left ventricular systolic function or dilated cardiomyopathy 1, 2

Postpartum Resolution

All measurements of left atrial size return to baseline by 6-8 weeks postpartum, confirming the reversible nature of pregnancy-related cardiac remodeling. 5, 3 This complete resolution distinguishes physiological adaptation from pathological left atrial enlargement due to underlying cardiac disease.