Why Pregnant Women Develop Systolic Flow Murmurs
Pregnant women develop systolic flow murmurs primarily due to a 50% increase in circulating blood volume and cardiac output, which creates turbulent flow across normal cardiac structures without any underlying pathology. 1, 2
Hemodynamic Mechanisms
The physiologic basis for systolic murmurs in pregnancy involves several key cardiovascular adaptations:
Blood volume increases by approximately 50% during normal pregnancy, with cardiac output peaking between the mid-second and third trimesters 1, 2
Stroke volume increases substantially, accompanied by a smaller elevation in heart rate (averaging 10-20 beats per minute) 1, 2
Systemic vascular resistance falls due to effects of uterine circulation and endogenous hormones, resulting in a hyperdynamic circulatory state 1
The combination of increased flow velocity through normal valve orifices and increased stroke volume creates turbulent blood flow that generates audible murmurs 3, 4
Clinical Characteristics of Normal Pregnancy Murmurs
The typical systolic flow murmur in pregnancy has specific features that distinguish it from pathologic murmurs:
A soft grade 1-2 midsystolic murmur is the most frequent finding, best heard along the mid to upper left sternal edge 1, 2
The murmur reflects increased flow across the pulmonary and aortic valves in the setting of elevated cardiac output 5, 3
These murmurs typically appear around 10-12 weeks gestation when hemodynamic changes become pronounced 5
The hyperkinetic precordial impulse accompanies the murmur due to increased contractility and stroke volume 1, 2
Additional Innocent Murmurs in Pregnancy
Beyond the typical systolic flow murmur, pregnant women may develop other benign murmurs:
Venous hum: A continuous murmur best heard in the right supraclavicular fossa that can be obliterated by chin movement toward the stethoscope or digital pressure over the ipsilateral jugular vein 1, 6
Mammary souffle: A systolic or continuous murmur over engorged breast tissue, heard in late pregnancy or early puerperium, that disappears with firm pressure on the stethoscope diaphragm or when standing 1, 6
Critical Distinction: When Murmurs Are NOT Benign
Diastolic murmurs are unusual in normal pregnancy and require further evaluation, as they typically indicate underlying valvular pathology rather than physiologic changes 1, 2
Important caveats to recognize:
While systolic flow murmurs are benign, pregnancy can accentuate murmurs from pre-existing stenotic lesions (mitral stenosis, aortic stenosis) due to increased blood volume and cardiac output 1
Conversely, murmurs of regurgitant lesions (aortic regurgitation, mitral regurgitation) may attenuate or become inaudible as systemic vascular resistance decreases 1
Any patient with symptoms beyond normal pregnancy (significant dyspnea, chest pain, syncope) or signs of heart failure requires comprehensive evaluation regardless of murmur characteristics 2
Structural Cardiac Adaptations
The heart undergoes remodeling to accommodate increased hemodynamic demands:
Mild ventricular chamber enlargement occurs as a normal adaptation 1, 7
Myocardial hypertrophy develops, particularly evident in end-systolic wall thickness measurements 7
Physiologic tricuspid and pulmonic regurgitation becomes common on Doppler echocardiography, though this rarely produces audible murmurs 1
The key clinical point is that echocardiography is not routinely indicated for isolated soft systolic murmurs in pregnancy without other clinical or ECG abnormalities, as these studies rarely alter management 5