Correlation of ultrasonic epiphyseal centers and the lecithin: sphingomyelin ratio.

Ossification centers about the fetal knee were identified and measured by ultrasound in 133 nondiabetic patients who underwent amniocentesis for determination of the amniotic fluid lecithin:sphingomyelin ratio (L:S ratio) between 28 and 42 weeks' gestation. A distal femoral epiphysis measuring greater than or equal to 5 mm and a proximal tibial epiphysis measuring 3 mm or greater was seen at 34 to 35 weeks' gestation. A mature lecithin: sphingomyelin ratio (lecithin: sphingomyelin greater than or equal to 2.0) was present with 66, 94, and 100% of proximal tibial epiphysis measuring 0 to 2, 3 to 4, and 5 to 7 mm, respectively. A mature lecithin: sphingomyelin ratio was present with 39, 76, and 95% of distal femoral epiphysis measuring 0 to 2, 3 to 4, and 5 to 7 mm, respectively. These results suggest that in nondiabetic patients, the ossification centers about the fetal knee as measured by ultrasound correlate well with amniotic fluid lecithin: sphingomyelin ratio.