Plasma estriol in the evaluation of third-trimester gestational age.

This study attempts to confirm previous reports of a clinically useful serum unconjugated estriol surge at 36 weeks' gestation. Although an apparently physiologic estriol surge occurred at 36 +/- 2.1 weeks in 25 of 32 patients, clinical reality makes weekly plasma sampling difficult. In individual cases, use of the "surge point" predicted gestational age within a 4-week range with only 66% accuracy, and potentially serious errors in dating occurred. Other biochemical (lecithin:sphingomyelin, phosphatidylglycerol) and sonographic methods are superior in resolving problems with dating gestational age in the third trimester.