Normal right and left ventricular mass development during early infancy.

The average newborn's right ventricular mass, indexed to body surface area, is approximately 20% above the adult level, consistent with in utero physiologic hypertrophy. In contrast, the left ventricular mass index is underdeveloped by about 30% at birth, resulting in a ratio of right to left ventricular mass that is nearly 75% higher than at maturity. Left ventricular mass index increases rapidly during the first 2 weeks of life and then more slowly thereafter, whereas the right ventricular mass index decreases steadily during the first 4 months of life.