Urinary estrogens during late pregnancy: observations on the validity of "24-hour" urine collections and the estrogen/creatinine ratio.

The determination of estriol content of the maternal urine in late pregnancy has gained widespread use as a means of assessing the fetoplacental unit. Major drawbacks have included the time lag in obtaining a 24-hour urine collection and obtaining an accurate 24-hour collection. To overcome these problems, it has been suggested that the estrogen/creatinine (E/C) ratio of incomplete collections or even random urine specimens might provide adequate information. Our analysis of these E/C ratio technics, as compared to accurate 24-hour urine estrogen values and fetal outcome, shows the E/C ratio to be less reliable and occasionally misleading.