In this study, we demonstrated that the composition of maternal gut microbiota during pregnancy was significantly different between pregnant women with FGR and normal controls. And the altered FGR-related microbial community was characterized by the increased abundance of genus Bacteroides, Faecalibacterium, Lachnospira. These findings might provide novel insight into the prevention and treatment of FGR.
Several indexes including ACE, Chao, Shannon and Simpson were used to profile the maternal gut microbiota from different aspects. Despite the lack of significant difference in these indices, PCoA plot revealed complete segregation of the FGR and control group. Furthermore, the differential relative abundance of specific taxa was presented in the two groups. We found that the relative abundance of phylum Firmicutes was significantly higher in the FGR group than that in the control group. Previous studies also found similar microbiota dysbiosis in pregnant women with GDM  and pregestational overweight and obesity . At genus level, Bacteroides was found to be increased in the FGR group. In support of our results, Bacteroides was significantly higher in neonates born to overweight mothers than that delivered to normal weight mothers . Other studies demonstrated that increased Bacteroides was associated with overweight and obesity in both adults [24–26] and pregnant women , which could increase the risk of fetal growth restriction and sudden intrauterine unexplained death [27, 28]. Moreover, in this study, Faecalibacterium and Lachnospira were also enriched in the FGR group. This is in agreement with results reported by Zacarias et al. that similar alterations were found in the overweight pregnant women compared to the normal ones . In general, we found altered maternal microbiota in pregnant women with FGR, which was consistent with dysbiosis occurred in various disorders during pregnancy.
It is well-known that obesity is associated with a state of chronic low-level inflammation . Reactive oxygen species (ROS) production is elevated in obesity, which causes enhanced activation of inflammatory pathways [30, 31]. Interestingly, Xu et al. reported that ROS are involved in lipopolysaccharide-induced intrauterine FGR in mice . According to previous studies, a higher F/B ratio was associated with an aggravation of low-grade inflammation and to a more elevated capability of harvesting energy from food . Bacteroidetes, a type of gram-negative bacteria, is the main contributor to LPS biosynthesis. Therefore, high abundances of Bacteroidetes may induce increased inflammation during pregnancy . Maternal LPS exposure at late gestational stages results in intrauterine FGR in mice [32, 35]. A recent study indicated that the level of Lachnospiraceae correlated negatively with energy consumption and positively with leptin level . In addition, Florencia et al. demonstrated inflammatory biomarker (high-sensitive CRP) values were correlated with several microbiota components, such as Lachnospiraceae and Faecalibacterium . Taken together, the over-represented Bacteroides, Faecalibacterium and Lachnospiraceae in FGR group might contribute to the development of FGR.
The greatest strength of our study is the homogenous characteristics of enrolled FGR cases. Placental disorders or umbilical cord abnormalities were the only causes of FGR among the participants, excluding maternal-fetal pathologies such as PE, diabetes, or fetal abnormalities. This reduced the confounding in microbiome data caused by heterogeneity in causes of FGR. In additionally, an EFW below the third percentile was adopted as the threshold of diagnosis of FGR in our study, thus allowing us to avoid including constitutionally normal newborns. FGR is often confused with small for gestational age (SGA) in clinical practice. And it is well known that lower growth percentile is associated higher likelihood of FGR and thus susceptibility to problems after birth . Another strength is that we strictly controlled for sterile conditions during sampling. Considering that the fecal samples are usually expelled and collected in toilet, microbes may be contaminated during this process. In contrast, all samples in this study were directly obtained from maternal rectum in the operation room by the same senior obstetrician according to the principle of sterility, which minimized the possibility of microbial exposure and colonization in vitro.
However, several potential limitations need to be taken into consideration. Firstly, the sample size was relatively small and all the participants were recruited from the same hospital, thus we could not completely rule out the potential regional differences in maternal gut microbiota. The reliability of current results would greatly benefit from larger FGR and control cohorts. Secondly, we were not able to record detailed information on diet and lifestyle of the mothers during pregnancy, which have also been shown to alter the microbiome. Therefore, the associations of dietary intakes and the altered FGR-related microbial community were not analyzed in this study. Therefore, the mechanism by which alterations of maternal microbiome induce FGR should be further explored in animal experiments with well-controlled feeding conditions. Moreover, short-read 16S rDNA amplicon sequencing technique limited our ability to examine gut microbiota at species and strain level, which requires deeper taxonomic profiles from metagenomic shotgun sequences.
To our knowledge, this is one of the earliest studies to characterize the maternal gut microbiota in pregnant women with FGR. Our results indicated a relationship between maternal dysbiosis during pregnancy and the risk of FGR, which might involve the dysregulation of glycometabolism. Since gut microbiota profiles are alterable through various means (e.g., probiotics and dietary changes), our findings could provide novel insights into the prevention and treatment of FGR.