Spontaneous preterm birth (sPTB) is a major cause of perinatal morbidity and mortality, even in developed countries. Prediction of sPTB is therefore a valuable tool to reduce the associated risks. The current standard for the prediction of sPTB consists, in addition to anamnestic data, of previous sPTB and previous second trimester miscarriage, measurement of cervical length by transvaginal ultrasound (TVU CL) together with assessment of fetal fibronectin levels in cervicovaginal fluid. Other evaluation parameters, such as the level of endocannabinoids in the pregnant woman's blood, could increase the sensitivity of this management. Endocannabinoids (eCBs) are a part of the endocannabinoid system (ECS); out of them anandamide (arachidonoyl-ethanolamide, AEA), in particular, plays an important role in the regulation of pregnancy and childbirth. We present the protocol for an open, non-randomized study to evaluate concentrations of AEA and other endocannabinoids: 2-linoleoylglycerol (2-AG), 2-linoleoylglycerol (2-LG), 2-oleoylgly-cerol (2-OG), and 2-arachidonoyldopamine (2-ADOPA or also NADA) in the blood of pregnant women as potential predictors of sPTB. In a total of 230 women with a history of sPTB or miscarriage, eCBs levels between 22 and 28 weeks of gestation will be assessed from maternal blood, in addition to the standard procedure. The aim of the study is to determine the relationship between blood concentrations of the endocanna-binoids tested and the risk of sPTB. The results of this study will describe the prognostic significance of maternal blood eCBs levels for sPTB, and could subsequently enable improved screening programs for early identification of sPTB.