Fetal growth restriction (FGR), also termed intrauterine growth restriction (IUGR), is a state that reflects the failure of a fetus to attain its full genetic growth at a particular gestational age. This means that the baby weighs less than 9 out of 10 babies of the same gestational age ^[1][2][1,2]. FGR is diagnosed using ultrasound and described by an estimated fetal birth weight (EFW) or abdominal circumference (AC) below the threshold of the 10th percentile for gestational age ^[1][2][1,2]. FGR is often used interchangeably with small for gestational age (SGA) (determined as a weight/length of less than two standard deviations under the mean for gestational age) but they are not the same, as SGA has the clinical features of malnutrition and in utero growth retardation ^[1][3][4][1,3,4]. SGA infants may have FGR, but SGA cannot be used as a proxy for FGR, and not all SGA infants are pathologically growth-restricted [5].

FGR is classified into two categories: symmetric and asymmetric. Symmetric FGR (hypoplastic small for date) is identified in early pregnancy in about 20% to 30% of FGR cases, and is characterized by a reduction in the size of all organs, resulting from fetal nutrient restrictions, poor placental function, and autosomal chromosome aberrations (e.g., aneuploidy). TORCH infections (including toxoplasmosis, rubella, cytomegalovirus, and herpes) are associated with symmetrical FGR. Asymmetric FGR (malnourished babies) manifests in the late second/third trimester of pregnancy in a large fraction of FGR cases (70–80%) and is caused by preeclampsia (PE) due to utero-placental insufficiency. Asymmetric FGR is characterized by a reduced abdominal circumference (fetal liver), while the head circumference (fetal brain) is of average size. The decrease in liver size results in a depletion of adipose tissue and reduced glycogen storage, as well as total blood flow to the fetus ^[2][3][4][6][2,3,4,6]. Infants with symmetric FGR have poor prognosis compared to those with asymmetric FGR. In symmetric FGR, the body’s cell numbers are usually abnormal and reduced in early pregnancy, which leads to fetal growth abnormalities ^[2][3][6][2,3,6].

The etiology of FGR is related to a myriad of factors. The maternal factors include low socioeconomic status, smoking, alcohol, antineoplastic agents, PE, anemia, hypertensive disorders, systemic lupus erythematosus, chronic renal disease, antiphospholipid syndrome, respiratory diseases, cystic fibrosis, gastrointestinal diseases, and infections ^{[1][2][3][4][6][7][8][9][10]}[1,2,3,4,6,7,8,9,10]. There are several factors related to the placenta. These include low placental weight, small terminal villi, infections, and confined placental mosaicism. Fetal anomalies, including chromosomal abnormalities, genetic syndromes, congenital anomalies/infections, and metabolic disorders, also play a role in FGR ^{[1][2][3][4][6][7][8][9][10]}[1,2,3,4,6,7,8,9,10]. Epigenetics may also contribute to FGR, which involve changes in gene expression resulting from DNA methylation, non-coding RNAs, and post-translational modification of histones. Environmental factors (e.g., smoking, alcohol) have been found to shape these epigenetic changes, in which many FGR-related gene expressions are involved in fetal phenotype and placental epigenome alterations [11].

FGR increases insulin sensitivity and decreases counter-regulatory hormones, intestinal perfusion, subcutaneous fat stores, and the body’s protein/nitrogen contents. Infants with FGR have shown a significant reduction in the uptake of nutrients (e.g., minerals, amino acids, glucose), likely as a result of placental insufficiency, which, in turn, can lead to chronic hypoxia and reduce glycogen stores in the muscles and liver ^[2][3][2,3]. FGR infants are at a significant risk of developing health consequences later in life. These include cardiovascular (e.g., coronary disease, heart failure, early-onset atherosclerosis), metabolic (e.g., fatty liver disease, type 2 diabetes, dyslipidemia), and respiratory consequences (e.g., asthma, impaired lung function, restrictive pulmonary disease) ^[3][12][13][3,12,13]. Oxidative stress (OS) and dysregulated genes related to inflammation have been postulated to explain the increased risk of developing these diseases ^[12][14][12,14]. FGR and its associated diseases are influenced by multiple molecular mechanisms, including transforming growth factor β (TGF-β), protein 53 (p53) phosphorylation, the mechanistic target of rapamycin (mTOR), nucleotide oligomerization domain receptor 1 (NOD1), heat-shock proteins, glucocorticoids, and leptin [14].

The early detection of pregnant women at high risk of developing FGR would allow the application of promising treatment strategies to improve fetal growth. Several randomized controlled trials (RCTs) have shown that FGR-specific treatments, such as maternal micronutrient and amino acid supplementation (iron, zinc, calcium, magnesium, N-acetyl cysteine/L-Arginine), maternal nitric oxide supplementation, maternal growth hormone supplementation, aspirin, antiplatelet/anticoagulant agents, calcium channel blockers, proton pump inhibitors, and melatonin/heparin, can improve birth weight in FGR pregnancies ^{[15][16][17][18][19][20][21][22][23][24]}[15,16,17,18,19,20,21,22,23,24]. These treatments appear to largely reflect the current understanding of the etiology of this condition.

Complicated pregnancy is a state where aberrant inflammation can occur due to an imbalance between pro- and anti-inflammatory cytokines/chemokines ^[25][26][27][25,26,27]. Aberrant inflammation can lead to an increased risk of FGR and other adverse pregnancy outcomes such as obesity, inflammatory bowel disease (IBD), gestational diabetes (GDM), PE, pregnancy loss, and autoimmune diseases [27]. Pregnancy is a period where the infant gut microbial composition undergoes changes influenced by several factors, including gestational age, lactation stage, maternal diet, antibiotic exposure, and mode of feeding/delivery ^[28][29][28,29]. Gut microbiota dysbiosis during pregnancy is a potential contributing factor to metabolic inflammation and has been associated with many diseases in pregnant women and infants, including endometriosis, environmental enteric dysfunction, PE, GDM, intrahepatic cholestasis, hyperemesis gravidarum, necrotizing enterocolitis (NEC), neonatal diabetes (NDM), obesity, and asthma ^{[30][31][32][33][34][35][36][37]}[30,31,32,33,34,35,36,37]. Gut-derived microbial components such as lipopolysaccharides (LPS) have been proposed as an underlying mechanism for this effect ^[38][39][38,39].

Probiotic supplementation has been shown to modulate aberrant gut inflammation in the early years of life, thereby providing them with a potential role in the treatment of inflammation-related diseases ^{[30][31][32][33][36]}[30,31,32,33,36]. Probiotic strains, such as short-chain fatty acids (SCFAs) producing Bifidobacterium and Lactobacillus, may have the potential to improve immune responses and alleviate inflammation by reducing the production of pro-inflammatory cytokines in infant intestinal epithelial cells ^{[30][31][32][33]}[30,31,32,33]. A recent review of RCTs suggests that probiotic supplementation may offer beneficial effects in reducing the risk of PE in pregnant women [40]. However, no comprehensive reviews on gut microbiota community profiles, inflammatory markers, and the potential impact of probiotic use in FGR are available.  Thus, this review aims to summarize the existing human and animal studies on gut microbiota and inflammatory profiles linked to FGR and the effect of probiotics as a potential therapeutic approach.

2. Effects of Probiotics in Fetal Growth Restriction

Possibly reflecting the limited number of observational studies in animal models and humans, the efficacy of probiotic therapies in ameliorating FGR in newborns and mothers seems to have been reported in only a few studies ^{[41][42][43][44][45][46]}[54,80,81,82,83,84]. The level of heterogeneity in study design also appears to be similar to the observational studies reviewed. The targets of intervention seem to more or less follow findings from the observational studies. These include stimulating intestinal development in the FGR offspring to increase digestion and absorption capacity and immune function ^[41][42][54,80], and regulating liver function ^[43][81]. Another approach aimed to restore fetal–placenta development in the pregnant mother to ameliorate alterations in the growing environment for the fetus ^[43][81]. Both approaches aimed to promote fetal growth and development.

2.1. Animal Models

Two animal models, one swine ^[41][54] and one rodent ^[42][80], were found to investigate the efficacy of probiotics in newborn growth and intestinal development. One murine model focused on maternal responses in terms of body composition, major metabolic and immune organs, caecum metabolites, and hormone levels in blood, in addition to fetal liver function ^[43][81]. The Bacillus subtilis PB6 strain used in a swine model ^[41][54] and the two breast milk-derived Lactobacillus strains Limosilactobacillus oris ML-329 (ML-329) and Lacticaseibacillus paracasei ML-446 (ML-446) administered to a rodent model ^[42][80] demonstrated their viability and increased abundance in the gut lumen. Both studies observed significant morphological and physiological improvement in the intestine in FGR neonates. The intestinal length and weight of FGR rats supplemented with ML-329 and ML-446 were superior to their corresponding blank controls ^[42][80]. Increased villous height ^[41][42][54,80] and reduced villi crypt ratio ^[41][54] were also reported. The latter indicates a better capacity for digestion and absorption. The accompanying physiological improvements, such as elevated maltase and sucrose activity levels in FGR piglets irrespective of body weight, added to the efficacy of the B. subtilis PB6 strain. Improved intestinal development was also accompanied by an augmented feeding efficiency, as represented by a significant reduction in feed conversion ratio, but not on body weight and organ-related growth parameters. The effects on improving intestinal barrier function were highlighted by elevated expression of genes for proteins crucial to intestinal tight junctions such as zonula occludens-1 in both FGR and control newborn piglets. The rodent model revealed that the aforementioned breast milk-derived Lactobacillus strains were able to stimulate the expression of genes involved in the Wnt signaling pathway and suppress those expressed in the Notch signalling pathway, and thus promoting the proliferation and differentiation of intestinal epithelial cells ^[42][80]. Among them, the mucin-producing goblet cells and the immunoregulatory Paneth cells are critical in maintaining intestinal barrier integrity and innate immunity ^[47][48][85,86]. These two animal models demonstrated the possibility that impaired intestinal development could be at least partially rescued by administrating probiotic strains known for their efficacy in stimulating gut tissue and immune function development. The possibility to improve fetal growth by promoting placenta development was underlined by findings from a murine model which was supplemented with the strain Bifidobacterium breve UCC2003/pCheMC following a pattern that took advantage of the natural elevation of Bifidobacterium sp., during pregnancy ^[43][81]. Maternal body composition and the subsequent organ responses to supplementation were modified, accompanied by increases in caecum and/or placenta carnitine and acetate. The former could affect energy metabolism and the latter is known to mediate host glucose metabolism and intestinal epithelial cell immune function. These changes were postulated to enable resource allocation to fetus. The reduction in the thickness of the interhaemal membrane barrier of the placenta could also contribute to fetal growth by diminishing the barrier to exchange resources between the placenta and the fetus. Further facilitating the maternal supply of resources to the fetus, the expression of genes involved in glucose and fatty acid transporters in placenta was augmented. These preliminary data seem to support the supplementation of probiotic strains during pregnancy at risk of FGR. Considering the lack of statistically significant improvement in newborn body weight reported by those animal models reviewed, studies supplementing multiple strains with unique verified effects on maternal energy metabolism and inflammatory status, placenta development, and fetal/infant intestinal development may help determine the clinical potential of probiotics therapy in FGR management ^[43][81]. Appropriately labelled probiotic strains or their metabolites may help confirm the cause and effects and/or trace the pathways involved in mediating the effects in these studies ^[43][81]. Findings from animal models on probiotic supplementation in FGR treatment are listed in Table 1. ^45][83] was administered to prevent antibiotics-associated gut complications, in the other an unspecified L. rhamnosus GG strain (0.5 × 10⁹ CFU/day) ^[46][84] was administered to prevent NEC. Sepsis with bacteria identified to match the supplemented strain was diagnosed in both cases ^[45][46][83,84]. Bacterial translocation across the intestinal barrier in severely ill FGR infants was suspected in both cases. While neither case identified the exact route of infection, the more recent case highlighted the possibility of contamination of the central venous catheter which contained glucose that potentially contributed to bacterial growth ^[45][83]. The establishment of safety protocols for prescription and administration of probiotic products in those suspected of impaired immune function including FGR infants, particularly those with glucose infusion, are warranted ^[45][46][83,84]. Table 2 summarizes findings from human studies that have evaluated the effectiveness of probiotic supplementation in FGR treatment.

2.2. Human Studies

The consumption of probiotic foods may provide beneficial effects in reducing prematurity complications during pregnancy. However, whether probiotic supplementation can reduce FGR in humans remains an ongoing debate. Evidence from a prospective cohort study over a 9-year period suggests that probiotic milk intake during late pregnancy was associated with a decreased risk of PE. Probiotic intake during early pregnancy was associated with a reduced risk of preterm delivery ^[49][87]. Another cohort study has demonstrated the potential of milk-based probiotic products to reduce the risk of PE in primiparous pregnant women over a 6-year period ^[50][88]. Clinical studies in humans seem to demonstrate even less optimistic results in its efficacy ^[44][82] and risk of adverse effects ^[45][46][83,84] A double-blinded RCT in FGR and normal weight infants born before 33 weeks of gestational age reported limited clinical benefits of Bifidobacterium breve M-16V supplementation until the age of 37 weeks. The only improvement reported was that those with FGR who were supplemented were able to reach full feeds a couple of days earlier than the FGR placebos. This was despite the significant increase in fecal counts of B. breve among the supplemented group in comparison with those not supplemented irrespective of their FGR status. This outcome was among a number of parameters tested including a comparison of total fecal B. breve counts 3 weeks post supplementation between FGR and normal weight controls, and a number of different growth and clinical parameters. It is unclear whether increasing the sample size or altering the probiotic strains administered would lead to better outcomes ^[44][82]. In addition to limited clinical benefits, safety concerns regarding probiotic supplementation in FGR infants were cautioned in two case reports ^[45][46][83,84]. In one case, Lactobacillus rhamnosus GG (LGG) ATCC 53,103 (3 × 10⁹ cfu, once a day) ^[ Current evidence from experiments in animal models and human trials, albeit scarce, indicates the plausibility and potential of probiotic strains supplementation in either the newborn or the mother to ameliorate impaired growth in FGR. Supplementation targeting infant gut microbiota appeared to mediate an improvement in intestinal development and immune function and possibly feeding efficiency. Therapies provided during pregnancy showed promising efficacy in partially restoring fetus–placenta exchange and fetal growth environment, both critical to support fetal development. The minimal efficacy of reaching full enteral feed a couple of days earlier in the one human trial found implies the need for more sophisticated clinical trials addressing the multiple limitations in study design identified in both animal and human studies. The two case reports of probiotic-associated sepsis highlighted clinical caution and evidence-based guidelines required when adopting probiotic therapies.