An explanation of the low lecithin/sphingomyelin ratio of amniotic fluid in early pregnancy.

We have attempted to determine why the lecithin/sphingomyelin (L/S) ratio of amniotic fluid is lower than 2 during an early stage of pregnancy. We found that, at 16 wk of gestation, long before the fetal lung secretes lecithin into the amniotic fluid, the L/S ratio was about 1. High-density lipoprotein isolated from the amniotic fluid also had such a low L/S ratio. The L/S ratios of the high-density lipoprotein from umbilical cord blood and maternal blood, however, were much higher, viz. 3.7 (+/- 0.25) and 6.4 (+/- 0.33), respectively. The increase coincided with a decrease in their fluorescence polarization. We suggest that the low L/S ratio of 16 wk amniotic fluid is caused by lipolysis of its lecithin, which is derived from fetal or maternal high-density lipoproteins.