Study objective: The aim was to quantify the changes in placental blood flow and combined ventricular output with gestational age in normal human fetuses to determine the percentage of total cardiac output that placental blood flow represented, and whether this changed with age.
Design: Two dimensional echocardiographic images of the umbilical vein, the proximal aorta and proximal main pulmonary artery were obtained, and cross sectional areas calculated from vessel diameters. Doppler velocity signals were recorded from each vessel and digitised to obtain velocity-time integrals. Placental blood volume flow and combined ventricular output were calculated as the products of flow velocity time integrals and cross sectional areas of the umbilical vein, and of the great arteries respective.
Subjects: Subjects were 64 normal human fetuses aged between 20 and 42 weeks gestation.
Measurements and main results: Placental flow and combined ventricular output both increased exponentially with gestational age (r = 0.79, and r = 0.84; both p less than 0.001). Placental flow correlated linearly with combined ventricular output (r = 0.69; p less than 0.01) and comprised almost one third of total cardiac output throughout the second and third trimesters.
Conclusions: These data describe the relationship between placental blood flow and combined ventricular output with age in the normal human fetus and provide a substrate by which placental insufficiency and resulting intrauterine growth retardation may be recognised early.