OBJECTIVE: Our purpose was to investigate the biochemical composition of the coelomic fluid in first-trimester;anembryonic pregnancies. STUDY DESIGN:Coelomic and maternal serum were obtained from 17 anembryonic pregnancies and 17 apparently normal pregnancies between 7 and 12 weeks of gestation that were matched for gestational age. Electrolytes, total proteins, urea, creatinine, lactate, and human chorionic gonadotropin concentrations and pH were measured. RESULTS: Significantly higher pH (p < 0.005) and bicarbonate levels (p < 0.001) and significantly lower total protein (p < 0.05), phosphate (p < 0.005), and human chorionic gonadotropin (p < 0.001) levels were found in coelomic fluid from anembryonic pregnancies compared with normal pregnancies. No difference was found for chloride, sodium, potassium, urea, creatinine, and lactate between the study and control groups. CONCLUSIONS: The fluid collected in anembryonic pregnancies is of exocoelomic origin and its biochemical characteristics reflect a failure of most placental biological mechanisms such as metabolic function, transport function, and endocrine activity.