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It appears that the human heart adapts to the transition to life outside the womb and during the first year of life by increasing the size of the ventricles. Researchers used echocardiography to periodically assess heart anatomy and function in 52 normal fetuses beginning as early as 16 weeks gestation and ending at 12 months old. Heart rate decreased during pregnancy and after birth without a change in ventricular ejection fraction and without a change in contractility. This suggests that the ventricles adapt by increasing in size.