Objective. To investigate the difference of prolactin (PRL) expression in peripheral blood and decidual stromal cells (DSCs) between unexplained recurrent pregnancy loss (URPL) and normal early pregnancy. Methods. Eighty cases of URPL patients in our hospital from March 2018 to March 2019 were recruited. Seventy women with induced abortion and 190 normal pregnancy women were selected as controls. Fasting peripheral blood and decidua tissues were collected. The level of PRL in peripheral blood was detected by enzyme-linked immunosorbent assay. Quantitative real time PCR (RT-qPCR), Western bloting (WB) and immunohistochemistry (IHC) were used to detect the PRL expression of DSCs. Independent sample t test was used to compare the data of the two different groups, and one-way analysis of variance test was used to compare the data of the groups. Results. In the URPL group, 21 cases were excluded for chromosomal abnormality, and 59 cases were finally included. There were 70 cases in the induced abortion group. One hundred and seventy-seven cases in the normal pregnancy group completed follow-up, of which 157 cases were classified as final live birth in the normal pregnancy group, and the rest 20 cases subsequently had pregnancy loss, of which 19 miscarriages occurred in the first 10 weeks of gestation, classified as pregnancy loss in the normal pregnancy group. The PRL levels of peripheral blood in the final live birth in the normal pregnancy group and induced abortion group were similar, with no significant difference between the two groups in the same gestation, and they all increased significantly with advancing gestation (normal pregnancy with final live birth group: 5-5+6 week vs 6-7+6 week, P = 0.002; 6-7+6 week vs 8-9+6 week,P = 0.012;Induced abortion group: 5-5+6 week vs 6-7+6 week, P = 0.015; 6-7+6 week vs 8-9+6 week, P = 0.023). The levels of PRL in the peripheral blood of patients with URPL had no significant change with advancing gestation, and they were significantly lower than those of women with final live birth in the normal pregnancy group and induced abortion women in the same gestation from the 6th week (6-7+6week: P = 0.012, 0.024; 8-9+6 week: P = 0.015, 0.003). In addition, the levels of PRL in the peripheral blood of the women in the normal pregnancy group who later suffered miscarriage were similar to those in the URPL group of the same gestation. The PRL expression of DSCs in the induced abortion group increased significantly in both RNA and protein level with advancing gestation (5-5+6 week vs 6-7+6 week: RT-qPCR, P = 0.020; WB, P = 0.010; IHC,P = 0.030. 6-7+6 week vs 8-9+6 week: RT-qPCR, P = 0.011; WB, P = 0.034; IHC, P = 0.012). However, the increase of PRL expression of DSCs in the URPL group was delayed: the PRL expression of DSCs in women with 6-7+6week and 8-9+6 week of gestation was significantly lower than that in women accepting induced abortion of the same gestation (6-7+6 week: RT-qPCR, P = 0.012; WB, P = 0.014; IHC,P = 0.028. 8-9+6 week: RT-qPCR, P = 0.011; WB, P = 0.030;IHC, P = 0.026). Conclusion. PRL expression of the decidua is defective. The PRL expression of DSCs in URPL patients is deficient, and the lack of decidualization of DSCs may be related to the occurrence of URPL. The low level of PRL in peripheral blood after 6 weeks of gestation may indicate embryonic dysplasia. [ABSTRACT FROM AUTHOR]