Evaluation of human fetal urine as a source of amniotic fluid phospholipids

The accumulation of surface active material in amniotic fluid during gestation is assumed to result from lung fluid secretion through the trachea. Some animal studies, however, have indicated that virtually all of the tracheal fluid is swallowed, whereas little if any enters the amniotic cavity. Following these observations fetal urine has been considered by some authors as an alternative source of amniotic fluid phospholipids. However, the phospholipid content of human fetal urine has not yet been determined. We have determined the L/S ratio and lamellar body concentration in human amniotic fluid and fetal urine obtained during 3 term cesarean deliveries. While both the L/S ratio and lamellar body particle concentration in the amniotic fluid samples were equivalent to values reported in term pregnancies, no measurable lecithin or sphingomyelin peaks were demonstrated in the urine samples and very few, if any, particles were counted. The lack of similarity between determinants of surface activity in human amniotic fluid and fetal urine does not support a major contribution of fetal urine to the phospholipid content of amniotic fluid.