Hillemanns and coworkers established, that infants born after induced labor were heavier, on the average, than those from a control group; who were born after spontaneous labor. Since it was postulated, that a weight loss preceded the onset of spontaneous labor, the birth weight and length of 444 infants born after induced labor and 70 infants born via a primary cesarean section was compared with a control group of 1,028 uncomplicated births via spontaneous labor. Thirty seven post term infants were also included in the study. The birth weight and length of infants after induced labor or delivered by primary cesarean section significantly higher than those in the control group. The post term infants were also significantly longer, but their weight was not significantly lower, than that in the control group. The major differences could not be accounted for by varying periods of gestation nor could they be accounted for by the special conditions of induced labor or cesarean section. Therefore, the only possibility remaining is that a weight loss precedes spontaneous labor. This may possibly be explained by the reduction in the relative water content which begins before birth and continues through the newborn period or by a relative placenta insufficiency. The infant weight loss and loss of amniotic fluid after the thirtyeighth week of pregnancy refutes the theory, that birth is induced by a stretching of the uterine wall. The possibility is discussed, that the onset of labor is influenced by a corresponding relief from tension such as that, which occurs following a premature rupture of the fetal membranes.