The length of gestation is considered a key indicator of child health, and PTB is associated with poorer health outcomes in infants who have increased risks of short-term complications, mainly owing to the immaturity of multiple organ systems and neurodevelopmental disorders. Yearly, approximately 15 million infants are born preterm worldwide at a rate of 10.6%, with significant regional variations from 8.7% to 13.4%
[54], with a higher prevalence in low- and middle-income countries
[55]. Prematurity is the cause of about 35% of newborn deaths, and of 18% of children younger than 5 years old. Reducing PTB is part of the United Nations Sustainable Development Goal 3, target 3.2, which endeavours to end all preventable deaths of newborns and children aged under 5 years by 2030
[55].
Spontaneous PTB is associated with multiple, complex, and not completely understood causes of disease, but a leading cause of PTB is infection resulting from the microbial invasion of the amniotic cavity. A characteristic of acute chorioamnionitis is neutrophilic infiltration at the maternal–fetal interface, with ensuing inflammation. Commonly, acute chorioamnionitis is a result of ascending infection and recent studies suggest a link between vaginal dysbiosis, vaginal inflammation, and ascending infection. Less commonly, microbes can invade the maternal–fetal interface via the haematogenous route, e.g., Zika virus, CMV, and
L. monocytogenes, where they can cause placental villitis and severe fetal inflammation and injury
[56]. Although some studies found preterm labour to be associated with sterile intra-amniotic inflammation or infection
[57], this does not preclude the involvement of the microbiota in the vagina modulating systemic host responses that could trigger an inflammatory response in the amniotic cavity leading to PTB. Studies linking the vaginal microbiota to PTB have yielded mixed results, exposing the need for further investigations
[58].
A prospective study with 464 Caucasian-American women and 360 African-American women analysed the association between the vaginal microbiome at midpregnancy, race, and spontaneous PTB
[59]. In the Caucasian-American cohort, 375 women delivered at term and 89 preterm, and, in the African-American cohort, 276 women delivered at term and 84 preterm. The vaginal microbiomes of both cohorts were significantly different; the African-American women had higher microbial diversity, a greater abundance of
L. iners, and a lower abundance of
L. crispatus. The vaginal microbiomes of both groups of women were significantly associated with race, PTB, and maternal factors such as poverty, education, marital status, age, and douching. A higher
L. crispatus abundance in term controls was the main difference with the microbiota of women who delivered PTB. In
L. crispatus-dominated microbiomes, diversity was significantly lower than in
L. iners-dominated ones, suggesting that the former species is better at suppressing bacterial vaginosis-like microbiomes
[59]. This result is consistent with the observation that
L. iners-dominated vagitypes shift more often toward a diverse bacterial community structure than those where
L. crispatus is predominant
[60].
The diversity of taxa measured by the Shannon index or the inverse Simpson index was greater in samples from women who experienced premature delivery, as reported in other studies with women of various racial backgrounds
[61].
L. iners was predominant in women who delivered at term or preterm, with increasing abundance in successive trimesters. Next in relative abundance were
L. crispatus in the term group and
L. jensenii in the preterm group. Other taxa more abundant in the preterm group than in the term group were Lachnospiraceae BVAB1,
Prevotella cluster 2,
S. amnii,
Dialister cluster 51;
P. amnii, Clostridiales BVAB2; Coriobacteriaceae,
Dialister micraerophilus, and
Parvimonas.
Other Adverse Outcomes
The most frequent disease outcomes of
S. agalactiae colonisation during pregnancy are infant meningitis or sepsis, which are accompanied by high mortality risk. Other infrequent outcomes include stillbirths, maternal infections, and prematurity; however, given the wide prevalence of this genital infection, the potential annual burden of GBS-associated PTB has been estimated for the first time at 50,000 births, with a wide uncertainty range
[62].
A pre-pregnancy health examination program included 89 women whose pregnancy outcomes were followed up for 1 year. Vaginal swabs were collected, 16S rRNA genes were sequenced, and
M. hominis colonisation was confirmed by qPCR. Cox models were used to estimate the fecundability odds ratio (FOR) for women with
M. hominis [63]. The prevalence of
M. hominis was 22.47% (20/89) with a relatively low abundance. The Simpson index of the
Mycoplasma-positive group was significantly lower than that of the negative group (
p = 0.003), suggesting that microbiome diversity appeared to increase with
M. hominis positivity. The relative abundance of
M. hominis was negatively correlated with
L. crispatus (
p = 0.024), but positively correlated with
G. vaginalis,
A. vaginae, and
P. bivia (
p < 0.05 for all). The cumulative one-year pregnancy rate for the
Mycoplamas-positive group was lower than that in the negative group (58.96% vs. 66.76%,
p = 0.029). After controlling for potential confounders, the risk of pregnancy in the positive group was reduced by 38% when compared with the negative group
[63].
3.2. Longitudinal Studies
Longitudinal studies of the vaginal microbiome examine the abundance of the microbial taxa and their change throughout pregnancy in the context of a dynamic environment. Childbearing may be considered a pro-inflammatory condition in which the vaginal microbiota might possess immunomodulation properties based on the modification of bacterial species; in particular, lactobacilli seem to play a key role in this process
[32]. The vaginal bacterial composition of pregnant women exhibits considerable convergence across different populations, becoming less rich and diverse compared to the vaginal bacterial populations of non-pregnant women. During pregnancy,
Lactobacillus taxa become the predominant bacteria in the vagina in most women of various racial backgrounds, leading to a decrease in alpha-diversity
[30].
The vaginal ecosystem of 64 Caucasian women with normal pregnancies was characterised in the first, second, and third trimester. Advancing in the pregnancy, there was a significant decrease in cases of bacterial vaginosis and an increase in cases with normal microbiota. The relative abundance of lactobacilli increased from 50% in the first trimester, to 73.4% in the second trimester, and to 79.7% in the third trimester. This shift was associated with marked changes in the vaginal metabolome: several metabolites, such as lactate, glycine, phenylalanine, leucine, and isoleucine, ordinarily found in healthy vaginas, reached their highest concentrations at the end of pregnancy. Concomitantly, the abundance of microbiota present in bacterial vaginosis decreased throughout the pregnancy, and there was a progressive reduction in the levels of metabolites such as biogenic amines, alcohols, propionate, and acetate associated with dysbiosis. The levels of cytokines IL-6 and IL-8 were positively correlated, and their lowest concentrations were measured in the second trimester. A total of 19 women had a
Candida infection, with 10 cases throughout the pregnancy; associated with this infection were higher levels of IL-8, 4-hydroxyphenyllactate, choline, and O- acetylcholine, as well as a higher concentration of leukocytes
[64].
A prospective study with weekly sampling of the vaginal microbiome was conducted with 40 pregnant women, of which 11 delivered preterm. The vagitypes identified corresponded well with the five described for non-pregnant women. The taxonomic composition and diversity of the microbiota was generally stable in each individual with some vagitype transitions during the pregnancy; the
Lactobacillus-dominated vagitypes I, II, III, and V were more stable than those of vagitype IV. Pregnancies with vagitype IV exhibited a stronger association with PTB, at every time window during gestation.
G. vaginalis was strongly associated with PTB; a high
Ureaplasma abundance combined with a low abundance of
Lactobacillus was associated with PTB as well. Twenty-five women provided a postpartum sample; their microbiota indicated that delivery generally was accompanied by a significant, sudden, and durable increase in bacterial community diversity, albeit not in all cases
[65].
An exploratory longitudinal investigation of the vaginal microbiota composition of eight Mexican women with healthy pregnancies collected samples in the third trimester of pregnancy and, subsequently, at childbirth at term. The vaginal microbiota was dominated by the
Lactobacillus genus at both time points. There were no statistically significant differences in relative abundances, absolute read count, bacterial richness, community diversity, evenness, and beta-diversity between the third trimester of pregnancy and the time of childbirth. Nonetheless, compared to the third trimester of pregnancy, a trend was observed of higher absolute read counts for the genera
Gardnerella,
Faecalibaculum,
Ileibacterium, and
Lactococcus, and lower absolute read counts of
Lactobacillus spp. at childbirth, but these changes in absolute read counts did not result in significant statistical differences between the microbial populations at both times. The results suggest that the vaginal bacterial composition is stable, and
Lactobacillus genus is the dominant taxa in Mexican women’s vagina at the third trimester of pregnancy and at childbirth
[32].
4. Conclusions
In healthy pregnancies, the bacterial communities in the vagina have a relatively lower number of taxa than in non-pregnant women, and are dominated by a few species, namely, those of the genus Lactobacillus, although other taxa such as A. vaginae, G. vaginalis, and Prevotella spp. are also found in normal pregnancies. Interestingly, although the smaller number of taxa in individuals commonly results in a lower alpha-diversity, marked differences in vagitypes between individuals effect a higher beta-diversity during pregnancy. A relationship has been found between the vaginal microbiome and racial background; women of Hispanic or African ancestry harbour more anaerobic flora in their microbiota than women of Asian or Caucasian backgrounds who commonly have Lactobacillus spp. as the dominant taxa. Besides racial background, other factors that modulate the vaginal microbiota include maternal age, previous pregnancies, blood pressure, behavioural habits, and various environmental factors.
Epidemiological studies provide evidence that the urogenital microbiota is linked to obstetric diseases. A marker of pregnancy complications is the proliferation in the genital microbiota to the significant abundance or dominance of aerobes such as
Acinetobacter baumannii,
E. coli,
E. faecalis,
S. aureus, and
S. agalactiae [66][67], or anaerobes like
A. vaginae, Clostridiales BVAB 1-3,
D. microaerophilus,
G. vaginalis,
M. hominis,
P. timonensis, and
U. urealyticum.
Investigations on the changes induced in the vaginal bacterial populations by pregnancy in health and disease indicated that the vaginal microbiota is stable in the absence of infections. These studies confirmed that considerable changes in the vaginal community composition occur immediately following pregnancy, as well as postpartum.
The stability of the vaginal microbiome during a healthy pregnancy is such that alterations in the bacterial flora dominated by
Lactobacillus spp. reflect the status of various obstetric conditions and are predictive biomarkers for certain pregnancy-adverse outcomes. Thus, it has been proposed that the composition of the vaginal microbiome may be a useful prognostic indicator of preterm labour and serve as a monitoring tool for tocolytic treatment to prevent PTB
[68].
Nonetheless, much more needs to be learned about the pathogenicity and mechanisms of host defence to these micro-organisms. Future investigations will serve to elucidate the functional effect of the microbial communities and/or specific bacterial species on homeostasis and disease during pregnancy. A better understanding of the host–micro-organism interactions might reveal new opportunities for disease prevention, therapy, and improving women’s quality of life and overall health.
In addition, the complex gut microbiomes are involved in host immunity, metabolism, digestion, and the functioning of the nervous system, and are important for the health of the mother and child. During pregnancy, changes may occur naturally in the microbiomes of the oral cavity, intestine, and breast milk. Changes in the structure and composition of the gut microbiomes with an increase in the abundance of various genera of micro-organisms, e.g.,
Acinetobacter,
Actinobacter,
Klebsiella,
Rothia, etc., and a decrease in others, e.g.,
Bacteroides,
Bifidobacterium,
Eubacterium, etc., can manifest in pregnancy complications such as gestational diabetes, gestational obesity, pre-eclampsia, diseases of the digestive tract, and autoimmune disease. The relationships between imbalances of the maternal gut microbiomes and their physiological effects during pregnancy are starting to be elucidated, but many more investigations are required to provide a comprehensive picture that would serve to foster maternal and offspring health
[69].