Over and above the evaluation of fetal standard data, chest and lung measurements were performed in a prospective cross-sectional study with real-time ultrasound. The study group consisted of 610 patients between 12 and 41 weeks' gestation. Only uncomplicated pregnancies with carefully documented gestational age were included. Patients with multiple pregnancy, fetal anomalies, intrauterine growth retardation or macrosomia and patients with oligohydramnios were excluded. Transversal (BTTD) and sagittal (BTSD) diameter of the bony fetal thorax were measured as an outer-outer-measurement at the level of the atrioventricular valves; the thorax should be as circular as possible and at right angles to the fetal spine. Lung measurements (LD) were performed in the same plane in the direct extension of the long axis of the fetal heart. The circumference of the bony thorax (BTC) was calculated from the two thoracic diameters according to the formula BTC = (BTTD+BTSD)/2 x 3.142. For all parameters growth curves with 5., 50. and 95. percentiles were established. Growth of the bony thorax as well as the lung diameter was shown in a nonlinear function. Lung diameter and thoracic circumference were used to calculate the ratio of these two parameters with reference to the gestational age.