The most likely cause of this woman's murmur is a bicuspid aortic valve, the most common congenital heart abnormality. The characteristic finding of a bicuspid aortic valve is an aortic ejection sound associated with either a systolic or diastolic murmur. While the murmur associated with aortic stenosis usually radiates to the carotid arteries, the murmur of a nonstenotic bicuspid aortic valve may radiate to the apex. Fetal echocardiography is indicated if there is maternal cardiac structural disease because of the increased risk of fetal cardiac abnormalities, which is estimated to be in the range of 3% to 7%.

The altered hemodynamics of pregnancy may bring out murmurs not previously heard or may bring the pregnant patient to medical attention for the first time owing to the increase in systemic blood volume and cardiac output. Systolic murmurs are common during pregnancy. Most often these are ejection murmurs caused by increased flow through the right and left ventricular outflow tracts. The murmurs tend to be grade 1/6 or 2/6 midsystolic murmurs that do not radiate. Diastolic murmurs are not common.

The systolic murmur associated with hypertrophic obstructive cardiomyopathy generally increases with maneuvers that decrease preload, such as the Valsalva maneuver or change in position from squatting to standing.

A mammary souffle is a continuous murmur with a soft, humming quality. It is typically heard over the breast during late pregnancy and lactation and is thought to result from increased blood flow to the breast.

While the murmur of mitral valve prolapse is often late systolic and can be associated with a click, this click would not be described as an ejection click. The click associated with mitral valve prolapse is mid- to late systolic and is related to tensing of the chordae tendineae or valve leaflets. The click is responsive to changes in ventricular volume induced by posture or pharmacologic agents.

Functional murmurs, or physiologic murmurs, can occur in the absence of valvular pathology. An increase in cardiac output, as occurs in pregnancy, can result in physiologic ejection murmurs.

Given the findings on examination, it would be appropriate to obtain transthoracic echocardiography for this patient to identify the etiology of the murmur and the potential need for adjuvant screening to evaluate for associated aortopathy.