Fetal lung maturation involves a number of separate developmental processes. An ultrasonic technique is reported for grading the dynamic behavior of the right middle lobe or lingula in the second or third trimesters on ultrasound (US) study. Fetal lungs are stiff initially. Lung compressibility during diastole is typical after 36 weeks' gestational age, when phospholipid profiles of the amniotic fluid also indicate lung-tissue maturity. Compressibility also occurs earlier in the third trimester and has indicated, in preliminary observations on a small subgroup of infants delivered prematurely, a low risk of subsequent neonatal respiratory distress syndrome. High-speed magnification US study of fetal lung compressibility can be used to evaluate fetal lung development, possibly independently of gestational age and of conventionally measured lecithin/sphingomyelin values in amniotic fluid.