The limitation of the study is the size of the study groups. The study sample size is large enough to draw conclusions. However, a larger population would strengthen our findings. Another limitation is that the gestational age at which the placental tissue was collected varied considerably. The aim of the study was to establish the difference between PTB placentas and placentas of term births. While we showed that there was a correlation between CBR expression and pregnancy duration, a more detailed temporal analysis of the difference in CBR expression, e.g., at every week of gestation, would deepen our understanding of the role of the ECS in PTB.