Biparietal diameter as a predictor of fetal lung maturity

A correlation has been established between fetal biparietal diameter (BPD), amniotic fluid lecithin/sphingomyelin (L/S) ratio and optical density at 650 nm (O.D. 650). We have noted that if the BPD is greater than 9.2 cm, the L/S ratio and O.D. 650 will be greater than 2.0 and 0.15, respectively, in 90% of cases. We reviewed the literature on the complication rate of third-trimester amniocentesis under ultrasonic guidance and concluded that in the absence of good dating, a reliably obtained BPD greater than 9.2 cm indicates fetal lung maturity in cases where amniotic fluid studies cannot be performed.