In this multicenter study, we analyzed placental samples collected in the Obstetrics Department at the Provincial Hospital of Przemyƛl, the Department of Perinatology of the Medical University of Bialystok, and the Obstetrics Clinic of the Medical University of Lublin. The placentas were collected prospectively from women who gave birth between March 2004 and December 2012. The participants were divided into three groups. Group 1 consisted of women who experienced PTB and their placentas were assessed for the expression of the CBR1 and CBR1a; Group 2 included women who also delivered preterm and whose placentas were examined for CBR2 expression. Some participants were included both in Group 1 and 2. Group 3 was the control group, it comprised women who delivered at term, and their placentas were tested for the presence of all three receptors (CBR1, CBR1a, CBR2).