A total of 150 women were enrolled in the study, and 45 patients were subsequently excluded due to insufficient RNA quality in the placental samples. Ultimately, the study included 95 patients who experienced PTB, divided into two groups: 87 in Group 1 for the analysis of CBR1 and CBR1a expression, and 59 in Group 2 for the analysis of the CBR2. Notably, some of the patients were included in both groups. The control group consisted of 14 women who delivered at term, and their placentas were tested for the presence of all three receptors (CBR1, CBR1a, CBR2). The participant selection process is shown as a flow chart in Figure 3.