Introduction: During pregnancy, major changes occur in the mother's body; sometimes these changes are dangerous for both mother and the fetus. By recognizing these changes mother's pregnancy conditions could be managed. In this study, changes in blood cells and spleen tissue were investigated in female Wistar rats on the 10^th and 20^th days of pregnancy and the first week of lactation.
 
Materials & Methods: In this study, 54 rats (12 males and 42 females) were selected. After observing vaginal plug in female rats, 18 pregnant and 6 non-pregnant rats were selected randomly and divided into four groups of 6 as follows: the non-pregnant control group, 10^th day of pregnancy group, 20^th day of pregnancy group, and first week of lactation group. Blood samples were taken from all the groups. Spleen tissues were stained using the hematoxylin-eosin method for histopathological evaluation. The data were analyzed by one-way ANOVA and Duncan’s test.
Findings: Platelets on the 10^th and 20^th days of pregnancy showed a significant reduction compared to the control group (P<0.001). White cells and lymphocytes on the 10^th day of pregnancy showed a significant increase compared to the control group (P<0.05). Macrophages on the 10^th and 20^th days of pregnancy were significantly elevated compared to the control group (P<0.001). The diameter of white pulp on the 10^th day of pregnancy showed a significant increase compared to the control group (P<0.05).
 
Discussion & Conclusions: Pregnancy increases white blood cells and reduces platelets in pregnant rats. It also increases the diameter of white pulp and spleen tissue macrophages. These changes can be caused by alterations in mother's immune system.