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Constipation in gestating and lactating sows is common and the inclusion of dietary fiber may help to alleviate this problem. We investigated the effects of inulin (INU) and isomalto-oligosaccharide (IMO), two sources of soluble dietary fiber, on gastrointestinal motility-related hormones, short-chain fatty acids (SCFA), fecal microflora, and reproductive performance in pregnant sows. On day 64 of gestation, 30 sows were randomly divided into three groups and fed as follows: a basal diet, a basal diet with 0.5% INU, and a basal diet with 0.5% IMO. We found that INU and IMO significantly modulated the levels of gastrointestinal motility-related hormones, as evidenced by an increase in substance P (P< 0.05), and a decrease in the vasoactive intestinal peptide concentrations (P< 0.05), indicating the capacity of INU and IMO to alleviate constipation. Furthermore, IMO enhanced the concentrations of acetic, propionic, isobutyric, butyric, isovaleric, and valeric acids in the feces (P< 0.05). High-throughput sequencing showed that IMO and INU increased the fecal microflora α- and β-diversity (P< 0.05). Methanobrevibacterwas more abundant (P< 0.05), whereas the richness of Turicibacterwas lower in the INU and IMO groups than in the control group (P< 0.05). In addition, IMO significantly increased litter size (P< 0.05). Overall, our findings indicate that INU and IMO can relieve constipation, optimize intestinal flora, and promote reproductive performance in pregnant sows.