Table 4. Major findings of the included studies.

Study	Increased in GDM	Decreased in GDM	Associations between bacteria and host parameters	Author conclusion
Mokkala et al.2020 [45]	T1:	T1:	Holdemania filiformis: ↑glucose Alistipes shahii: ↓glucose Bifidobacterium bifidum: ↓glucose	The specific gut microbiota species do not contribute to GDM in pregnant women with overweight or obesity. However, the gut microbiota of GDM women were less responsive to the diet intervention.
Ma et al.2020 [46]	T1: Eisenbergiella, Lachnospiraceae NK4A136 group, Tyzzerella 4	T1: Dialister, eubacterium eligens group, eubacterium xylanophilum group, megasphaera, parabacteroides, parasutterella, Ruminococcaceae UCG 002, Ruminococcaceae UCG 003, Ruminococcaceae UCG 005	Eisenbergiella: ↑FG Tyzzerella: ↑FG Parabacteroides: ↓FG Parasutterella: ↓FG Ruminococcaceae UCG 002 ↓FG Dialister: ↓fasting insulin, ↓daily oil and yogurt intake.	The results demonstrated that aberrant gut microbiota interactions were associated with GDM before its onset, which was mainly reflected through the observed alterations in gut microbial composition and bacterial gene functions
Mokkala et al.2017 [47]	T1: Clostridia, clostridiales, Ruminococcaceae, Ruminococcaceae;g_unclassified, Ruminococcaceae;g_unclassified;s_unclassified	T1: Bacteroidia, Bacteroidales	Rumninococcaceae: ↑FG	The gut microbiota composition differs in women who developed GDM compared with women who did not develop GDM.
Gomez-Arango et al.2016 [48]	T1:	T1:	Actinobacteria: ↑insulin, ↑HOMA-IR Tnericutes: ↓insulin, ↓HOMA-IR Coriobacteriaceae: ↑insulin, ↑c-peptide Collinsella: ↑insulin, ↑HOMA-IR, ↑C-peptide, ↑maternal triglycerides, ↑VLDL cholesterol levels. Ruminococcaceae: ↑insulin, ↑leptin, ↓ fasting GIP, ↓resistin Coprococcus: ↑fasting GIP Lachnospiraceae: ↑leptin Bacteroidaceae: ↑ghrelin Prevotellaceae: ↓ghrelin	A relationship exists between the gut microbiome composition and the metabolic hormone milieu in early pregnancy.
Liu et al.2020 [40]	T1: T2: Faecalibacterium, blautia	T1: T2: Bacteroidetes, Akkermansia, butyricimonas, ChristensenellaceaeR_7group, odoribacter	Faecalibacterium: ↑2h OGTT, ↑IL-6, ↑TNF-α Akkermansia: ↓OGTT values Butyricimonas: ↓OGTT values Christensenellaceae R-7 group: ↓OGTT values Odoribacter: ↓OGTT values, ↓IL-8	There is an association between GDM and profound shifts in gut microbiota during T2. The specific bacterial patterns in the GDM women were correlated with blood glucose levels and inflammatory states.
Zheng et al.2020 [49]	T1: T2: Eggerthella, Holdemania, megasphaera	T1: Coprococcus, desulfovibrio, Intestinimonas, Peptococcus, prevotella, Streptococcus, Bacilli, Lactobacillales, Streptococcaceae, veillonella T2: Coprococcus, Flavonifractor, Streptococcaceae, Streptococcus	Did not detect any significant associations between microbial taxa and glucolipid measures, including fasting plasma glucose, lipid profiles, homeostatic model assessment-insulin resistance (HOMA-IR) score, and HOMA-cell index.	There are significant differences in the dynamics of gut microbiota from early to middle pregnancy between the groups. Women who develop GDM have reduced inter-time point variability in gut microbiota.
Koren et al.2012 [12]	T1: T3:	T1: T3:	NA	The study did not detect any differences between the microbiotas of GDM+ and GDM mothers.
Wang et al.2020 [50]	T2: Erysipelotrichaceae, prevotellaceae, Veillonellaceae, verrucomicrobiaceae, Akkermansia, allisonella, clostridium innocuum group, Lachnospiraceae UCG-010, prevotella 2, roseburia, terrisporobacter, Tyzzerella 3, unclassified_f_lachnospiraceae, lachnospiraceae, peptostreptococcaceae	T2: Enterococcaceae, Ruminococcaceae, enterococcus, faecalibacterium, intestinibacter, klebsiella, Lachnospiraceae_N2004_group, norank_f_ruminococcaceae, Ruminococcus2, unclassified_f_enterobacteriaceae, lachnospiraceae, peptostreptococcaceae	Klebsiella: ↑ FG at 12 wks. Ruminococcus_2: ↑ FG at 12 wks. Norank_f__Ruminococcaceae: ↓2h OGTT, ↑ FG at 12 wks. Lachnospiraceae UCG-010: ↑ 1h OGTT Roseburia: ↑ 1h OGTT Prevotella 2: ↑ 1h OGTT Enterococcus: ↓FG. ↓1h OGTT, ↓2h OGTT Lachnospiraceae_NC2004_group: ↓FG Akkermansia: ↓2h OGTT	GDM women have a significantly different microbial and metabolic signatures.
Chen et al.2020 [51]	T2: Bacteroidetes, bacteroidia, Betaproteobacteria, atopobium, bacteroides, butyricimonas, Campylobacter, Dialister, f_Rikenellaceae;g_unclassified, odoribacter, Sutterella, unclassified_f_enterobacteriaceae	T2: Actinobacteria, Firmicutes, actinobacteria, clostridia, Erysipelotrichia, Bifidobacterium, f_coriobacteriaceae;g_unclassified, f_ruminococcaceae;g_unclassified, f_ruminococcaceae;g_unclassified, f_veillonellaceae;g_unclassified, gemmiger, o_clostridiales;f_unclassified;g_unclassified, oscillospira, unclassified_f_lachnospiraceae	A module mostly of genera from firmicutes: ↓OGTT values A module of the genera veillonella, haemophilus and rothia: ↓FG Bacterial populations mostly of genera within the phylum Firmicutes (Gemmiger, Oscillospira, unassigned genera of Clostridiales, Ruminococcaceae and Lachnospiraceae): ↓ one or more OGTT values Atopobium: ↑glucose Sutterella: ↑glucose Unassigned genera of Enterococcaceae: ↑glucose	The study shows a relationship between changed gut microbiota composition in the second trimester of pregnancy before the diagnosis of GDM and fasting serum levels of metabolites.
*Kuang et al.2017 [52]	T2: Megamonas, parabacteroides, phascolarctobacterium, Bacteroides sp. 2_1_33B, Bacteroides sp. 3_1_19, Bacteroides sp. D1, Bilophila sp. 4_1_30, Capnocytophaga sp. oral taxon 338, Clostridium sp. L2-50, Coprobacillus sp. 29_1, Coprococcus comes, Lachnospiraceae bacterium oral taxon 082, Lactobacillus fermentum, Lactobacillus salivarius, Megamonas funiformis, Megamonas rupellensis, Parabacteroides distasonis, Parabacteroides goldsteinii, Parabacteroides sp. 2_1_7, Parabacteroides sp. 20_3, Parabacteroides sp. D13, Parabacteroides sp. D25, Paraprevotella clara, Phascolarctobacterium succinatutens, Streptococcus agalactiae, Streptococcus anginosus, Weissella confusa	T2: Clostridiales, coriobacteriaceae, aggregatibacter, eggerthella, fusobacterium, haemophilus, mitsuokella, roseburia, ruminiclostridium, Aeromicrobium massiliense, Alistipes finegoldii, Alistipes senegalensis, Alistipes shahii, Bifidobacterium adolescentis, Bifidobacterium bifidum, Eubacterium siraeum, Intestinibacter bartlettii, Methanobrevibacter smithii, Roseburia intestinalis, Roseburia inulinivorans	Parabacteroides distasonis: ↑2h OGTT Klebsiella variicola: ↑1h OGTT, ↑2h OGTT Eubacterium rectale: ↑1h OGTT Lachnospiraceae bacterium 2-1-58FAA: ↑FG, ↑1h OGTT Catenibacterium mitsuokai: ↑FG Alistipes shahii: ↓FG Bacteroides: ↓FG, ↓1h OGTT Methanobrevibacter smithii: ↓2h OGTT Tannerella sp. 6_1_58FAA_CT1: ↓1h OGTT, ↓2h OGTT Citrobacter freudii: ↓1h OGTT Eubacterium siraeum: ↓FG Eubacterium: ↓FG, ↓1h OGTT, ↓2h OGTT Alistipes senegalensis: ↓FG, ↓1h OGTT, ↓2h OGTT Eubacterium eligens: ↓FG Bacteroides sp. 4_1_36: ↓FG, ↓2h OGTT Eggerthella spp.: ↑ glucose tolerance Megamonas spp.: ↑ glucose tolerance Allofustis seminis: ↑ glucose tolerance Several species in Lachnospiraceae: ↑ glucose tolerance Several species in Parabacteroides: ↑ glucose tolerance Several Alistipes spp.: ↓glucose tolerance.	Women diagnosed with GDM suffered from moderate gut bacterial dysbiosis and functional dysbiosis that was not restricted to certain microbial species.
Li et al.2021 [54]	T3: Firmicutes, Clostridia, coriobacteriia, clostridiales, coriobacteriales, coriobacteriaceae, lachnospiraceae, Ruminococcaceae, blautia, collinsella, coprococcus, dorea, lachnospira, ruminococcus, blautia producta, clostridium spiroforme, collinsella aerofaciens, coprococcus catus, eubacterium dolichum, ruminococcus callidus, Ruminococcus gnavus	T3: pyramidobacter piscolens, Bacteroidetes, bacteroidia, Betaproteobacteria, Bacteroidales, burkholderiales, alcaligenaceae, dethlosulfovibrionaceae, pyramidobacter, Sutterella	Clostridium spiroforme: ↑FG Eubacterium dolichum: ↑FG Ruminococcus gnavus: ↑FG Pyramidobacter piscolens: ↓FG	This study showed a significantly difference in the gut microbiota between women with and without GDM in the third trimester of pregnancy.
Xu et al.2020 [53]	T3: Gammaproteobacteria, haemophilus, Pasteurellales, Pasteurellaceae	T3: Rikenellaceae, Alistipese, phascolarctobacterium	Bifidobacterium: ↓ maternal blood neutrophil counts, ↓ maternal white blood cell counts Ruminococcus: ↓ maternal blood neutrophil counts, ↓ maternal white blood cell counts Gemmiger: ↑ Neonatal body weight Akkermansia: ↓CRP	The maternal intestinal and oral microbiota at later pregnancy were significantly affected by GDM status.
Cui et al.2020 [55]	T3:	T3:	NA	The total faecal microbiota of healthy pregnant women and diseased pregnant women in the third trimester were similar, with no significant difference in gut microbiota.
Wu et al.2019 [56]	T3: Bacteroides dorei, Bacteroides sp. 3 1 33FAA	T3: Alistipes putredinis, Lactobacillus casei	Bacteroides dorei: ↑FG, ↑1h OGTT Alistipes putredinis: ↓1h OGTT, ↓2h OGTT	GDM women showed greater between-individual diversity compared to the control group.
Liu et al.2019 [57]	T3: Verrucomicrobiota, Akkermansia	T3: Faecalibacterium	Proteobacteria: ↑TC, ↑LDL Actinobacteria: ↓2h OGTT Faecalibacterium: ↓TG, ↑LdMePE, ↑LPEt, ↑PG Streptococcus: ↑TC Actinomyces: ↑TC Veillonella: ↑TC, ↓HDL Haemophilus: ↑TC, ↓HDL Firmicutes: ↑3-Dehydrocarnitine, ↑CER, ↑DG, ↑Hexadecanamide, ↑Octadecanoicacid, ↑PEt, ↑PIP3 Bacteroidetes: ↓3-Dehydrocarnitine, ↓CER, ↓DG, ↓cPA, ↓Hexadecanamide, ↓PEt, ↓PIP3, ↓PC Actinobacteria:↓SO Tenericutes:↓SM Prevotella: ↑LPG, ↓PIP3	GDM women had a lower diversity of the gut microbiota.
Cortez et al.2018 [58]	T3: Saccharibacteria, Gastranaerophilales, Christensenellaceae, Erysipelotrichaceae, lachnospiraceae, Peptococcaceae, Ruminococcaceae, Anaerostipes, bacteroides, Bifidobacterium, blautia, ChristensenellaceaeR_7group, Clostridiales-Family XIII AD3011 group, clostridium_sensu_stricto, collinsella, dorea, Eisenbergiella, Enterorhabdus, Lachnospiraceae NK4A136 group, Lachnospiraceae UCG-008, prevotella 9, roseburia, ruminiclostridium, ruminiclostridium 5, ruminiclostridium 9, Ruminococcaceae NK4A214 group, Ruminococcaceae UCG 002, Ruminococcaceae UCG 014, Ruminococcus1, Senegalimassilia, Subdoligranulum, Eubacterium coprostanoligenes, Eubacterium ruminantium	T3: Eubacterium rectale	NA	The results show a tendency toward dysbiosis in the GDM condition, characterized by the presence of certain pathogenic genera and decreased diversity.
Wang et al.2018 [59]	T3: Fusobacterium	T3:	Faecalibacterium/Fusobacterium: ↓2h OGTT	The microbiota of pregnant women and neonates were altered in GDM, with a strong correlation between certain discriminatory bacteria and the oral glucose tolerance test.
*Crusell et al.2018 [60]	T3: Granulicatella, Actinobacteria, actinobacteria, actinomycetales, coriobacteriales, coriobacteriaceae, leuconostocaceae, micrococcaceae, actinomyces, blautia, collinsella, desulfovibrio, leuconostoc, mogibacterium, Rothia, Ruminococcus2, faecalibacterium 8 mo. pp.: Actinobacteria, actinobacteria, coriobacteriales, clostridiaceae_1, coriobacteriaceae, Alistipese, Anaerovovorax, clostridium_sensu_stricto, collinsella, dehalobacter, hafnia, Howardella, olsenella, Phreatobacter, faecalibacterium	T3: Marvinbryantia, Oscillibacter, faecalibacterium, acetivibrio, Anaerosporobacter, bacteroides, butyricicoccus, clostridium IV, Clostridium XVIII, Erysipelotrichaceae_incertae_sedis, Intestinimonas, Isobaculum, Sutterella, veillonella 8 mo. pp.: Ruminococcus2, faecalibacterium, pseudomonadales, fusobacteriaceae, bacteroides, clostridium IV, eggerthella, fusobacterium, Isobaculum, Lachnospiracea_incertae_sedis	Collinsella: ↑FG Actinobacteria: ↑FG Butyricicoccus: ↓insulin sensitivity Prevotella: ↑ 2h OGTT Faecalitalea: ↑2h OGTT Verrucomicrobioales: ↓insulin sensitivity Verrucomicrobia: ↓insulin sensitivity Akkermansia: ↓insulin sensitivity Blautia (OTU_2383, 3654, 140, 2684): ↑2h OGTT Blautia (OTU_2383, 3654, 140, 2684): ↑FG, ↓insulin sensitivity, ↓disposition index Blautia (OTU_486): ↓FG, ↓2h OGTT, ↑Insulin sensitivity Escherichia/Shigella (OTU_680, 361, 3): ↑disposition index, ↓insulin sensitivity, ↑FG, ↑2h OGTT Clostridium IV (OTU_68): ↓2h OGTT Christensenella (OTU_63): ↑FG, ↓weight gain Bacteroides (OTU_4999): ↑hsCRP Alistipes (OTU_98): ↓hsCRP Anaerovorax (OTU_538): ↓hsCRP Acetivibrio: ↓pre-pregnancy BMI Leuconostoc: ↓pre-pregnancy BMI Clostridiales(7 of 11 species): weight gain Alistipes (OTU_128): ↓weight gain Eisenbergiella (OTU_258): ↑Weight gain Lactobacillus (OTU_80): ↑weight gain	GDM diagnosed in late pregnancy is associated with an aberrant gut microbial composition at the time of diagnosis. About 8 months postpartum, the gut microbiota of previous GDM women is still different from women who had a normal pregnancy.
Fugmann et al. 2015 [61]	8 mo. pp.: Rikenellaceae, Veillonellaceae	8 mo. pp.: Firmicutes	NA	This study suggests that distinctive features of the intestinal microbiota are present in post-GDM women at risk for T2D.
Hasan et al.2018 [62]	5 years pp:	5 years pp:	NA	The study found no differences in the gut microbiota 5 years postpartum between women with and without GDM.
Hou et al. 2020 [63]	?: Actinobacteria, Bacteroidetes	?: Firmicutes	NA	GDM women had a different gut microbiota composition, but this was influenced by age.

Bacteria are illustrated if they are significantly different in relative abundance between cases and controls. ↑ indicates positively correlation between that bacteria and the host parameter, while ↓ indicates a negative correlation between the bacterium and the host parameter. Pp, postpartum; mo., months; T1, first trimester; T2, second trimester; T3, third trimester; FG, fasting glucose; HOMA-IR, Homeostatic Model Assessment for Insulin Resistance; VLDL, Very Low Density Lipoprotein; GIP, gastrointestinal polypeptide; IL-6, interleukin 6; OGTT, oral glucose tolerance test; TNF-α, tumor necrosis factor alpha; IL-8, interleukin 8; wks., weeks; CRP, C-reactive protein; TC, total cholesterol; LDL, low-density lipoprotein; LPEt, lysophosphatidylethanol; PG, phosphatidylglycerols; HDL, high-density lipoprotein; CER, ceramides; DG, diacylglycerols; PEt, phosphatidylethanol; PIP3, phosphatidylinositol 3; SO, sphingoshine; SM, sphingomyelins; LPG, lysophosphatidylglycerol; hsCRP, high-sensitivity C-reactive protein; cPA, cyclic phosphatidic acid; LdMePE, lysodimethylphosphatidylethanolamine.