The site of amniocentesis and the lecithin-sphingomyelin ratio.

Simultaneous sampling of amniotic fluid from two sites within the amniotic cavity proximal and distal to the fetal mouth was performed. The lecithin-sphingomyelin (L/S) ratio of amniotic fluid obtained close to the fetal mouth was usually higher than that of amniotic fluid obtained from a site distal to the fetal mouth (20 of 26 cases). Differences of up to 0.7 in the two L/S ratios were measured. This suggested that pooling of amniotic fluid phospholipids occurred and in cases of borderline pulmonary maturity the site of amniocentesis may have influenced the predictive value of the L/S ratio.