Full citations found in PubMed database 222 items Full citations found in PubMed database 191 items Total clearly not relevant citations based on the analysis of titles 31 items Potentially relevant to abstract analysis Articles excluded by the absence of the exercise group (n = 1) Articles excluded by the absence of variables related to placenta morphology   (n = 9) Articles excluded by drug intervention group (n = 1) Articles excluded by the use of animals (n = 11) 9 Items Potentially relevant to full data analysis Articles excluded by gestational age outcome analysis   (n = 1) Articles excluded by the absence of placental data   (n = 1) Articles excluded by the absence of control group or data (n = 2) Articles excluded for lack of data (mean and standard deviation) (n = 1) 4 items Included in the systematic review and meta-analysis