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vascular anastomoses in the second trimester of pregnancy.235 The effects on blood pressure and cardiac function in children of 10 years of age are reassuring with normal blood pressures recorded and no important abnormalities of cardiac function appreciated using echocardiographic methods.236,237 However, interesting differences in the resting state and responsiveness of the microcirculation between monochorionic and dichorionic twins have been reported, suggesting that vascular programming occurs even in monochorionic twins with balanced intertwin transfusion.238 Vascular Programming Following Fetal Coarctation Coarctation of the aorta can be diagnosed in the fetus Both an hypoplastic arch and a coarctation shelf can be visualized and quantified using ultrasound.35,239 A continuous Doppler pattern of flow in the isthmus may be recorded and reflects altered patterns of flow which suggest an obstruction (Video 6.13, Fig 6.25) Studies of endothelial function in normotensive young adults undergoing repair of the coarctation as neonates and infants have demonstrated reduced endothelial dependent and independent function confined to the precoarctation site compared with normal controls This implies that early alteration of flow in fetal life may program later function in spite of early and adequate surgical repair181,240 and provide a plausible explanation for hypertension in approximately one-third of adults who have undergone coarctation repair in infancy or childhood FIG 6.25 Continuous Doppler pattern of flow in the isthmus reflecting altered patterns of flow that suggest arch obstruction in fetal life and possible programming of the responses of the arterial tree in adult life Summary Noninvasive measurement of the developing circulation in the human fetus has become a reality Doppler ultrasound technology now enables serial examination of the human fetus from the first trimester, allowing us to observe noninvasively developmental changes in cardiovascular function, myocardial maturation, and the responses to structural and functional disease states More quantitative and automated measures of myocardial function, organ perfusion, volume flow, and ventricular volumes enable a better appreciation of cardiac output and organ flows in health and disease The developing arterial tree can be examined by pulse wave velocity and its endothelial responses measured The link between the uterine environment, maternal disease, and fetal cardiovascular responses and longer-term pathology such as hypertension is now better characterized Early detection of cardiovascular abnormalities may permit the initiation of interventional strategies before birth and in early childhood that could reverse the progression to clinically important disease