Objective — to determine the levels of biologically active substances (cyclic nucleotides and phospholipids) in the blood of a pregnant woman with threatened preterm labor for the optimization of its prediction and diagnosis. Subject and methods. The authors were followed up 180 pregnant women who were divided into 2 groups: 1) 50 women with physiological pregnancy (a control group); 2) 130 women with threatened preterm labor (a study group). A set of examinations of the pregnant women included standard clinical, laboratory, immunological, and hemostasiological instrumental studies and determination of cyclic nucleotides, such as cyclic adenosine-3,5-monophosphate (cAMP) and cyclic guanosine-3,5-monophosphate (cGMP), and a spectrum of serum phospholipids in the examinees. Results. The concentrations of cAMP and cGMP and their ratio in the serum of patients in the study group were significantly decreased in the second and third trimesters of pregnancy. Investigation of phospholipid levels established a significant increase in phosphatidylcholine, sphingomyelin, and phosphatidylserine and a decrease in lysophosphatidylcholine in the second trimester of pregnancy compared with those in the control group. In the third trimester, there was a significant reduction in the content of phosphatidylethanolamine compared to that in the control group. Conclusion. The metabolism of cyclic nucleotides and phospholipids changes in threatened preterm labor. It is appropriate to determine biologically active substances, such as cyclic nucleotides and phospholipids, which can assist in refining the prognosis, diagnosis, and management tactics in a pregnant woman with a compromised obstetric history of recurrent miscarriage or preterm birth.