Prospective assessment of early fetal loss using an immunoenzymometric screening assay for detection of urinary human chorionic gonadotropin**Supported by grants from the National Institute of Environmental Health Sciences Superfund Basic Research Program, Research Triangle Park, North Carolina (P42ES04699), the National Institute of Child Health and Human Development, Bethesda, Maryland (R01HD28632), and March of Dimes, White Plains, New York (15-105).