Main findings

In this systematic literature review and meta-analysis, reproductive tract non-viral mono-infections were associated with lower rates of adverse maternal and neonatal outcomes, including preterm delivery, neonatal intensive care unit admission, and neonatal death, compared to non-viral co-infections. However, the analysis did not reveal significant differences between the groups in terms of gestational hypertension, eclampsia, spontaneous abortion or stillbirth, and chorioamnionitis.

This study represents the most extensive systematic review and meta-analysis conducted to date of reproductive tract non-viral mono-infections and co-infections during pregnancy. Our findings indicate that co-infections may exacerbate certain neonatal outcomes while having a similar impact on maternal complications compared to mono-infections. However, the studies did not assess the potential long-term effects of bacterial infections on maternal health beyond the gestation period.

Despite the high incidence of reproductive tract bacterial infections in pregnant women and the well-documented risks they pose to newborns, ^{Reference Korenromp, Rowley and Alonso20–Reference Olaleye, Babah, Osuagwu, Ogunsola and Afolabi23} there is a notable lack of studies comparing outcomes between mono-infections and co-infections during gestation. Our analyses revealed statistically significant differences between the two groups only in the rates of prenatal delivery, neonatal death, neonatal intensive care unit admission, and low birthweight, all of which favored mono-infections over co-infections. Among analysis of specific infections, only preterm birth showed a statistically significant difference, favoring the chlamydia mono-infection group over the chlamydia co-infection group.

Strengths and limitations

To ensure comprehensiveness, we included studies regardless of the GA at birth, HIV serologic status, or the number of fetuses. Whenever mentioned by the study, we described these baseline characteristics. Stratified analyses could not be performed based on publication date, location, study design, diagnosis, treatment, birthweight, and GA, because of the number of studies. Even when analyzing all studies that reported each of the outcomes, considering all infections, the number of studies for each analysis was low (only two or three). Conducting stratified analyses based on demographic characteristics would have resulted in a negligible sample size, with only one study per analysis. To enable some of the analyses, we combined similar outcomes reported by different studies into composite outcomes, such as eclampsia and preeclampsia with severe features, or spontaneous abortion and stillbirth.

Our study has several limitations. First, the included studies were non-randomized, which inherently increases the risk of both measured and unmeasured confounding factors, such as maternal age, socioeconomic status, and underlying comorbidities. However, given the ethical challenges of conducting randomized prospective studies to assess the risks of co-infection and mono-infection, non-randomized retrospective designs were likely the most appropriate and feasible approach. Furthermore, adjusted ORs controlling for factors such as GA and HIV serological status were not possible due to the lack of individual participant data and inconsistent reporting across studies. This limitation is crucial because the GA at infection significantly affects outcomes. For instance, during the early stages of pregnancy, bacterial infections and antibiotic treatment can markedly increase the risk of miscarriage and birth defects; in the third trimester, preterm labor and premature rupture of membranes become the primary concerns. ^{Reference Hu, Huang, Cai, Xu, Jiang and Gao24,Reference Ailes, Gilboa and Gill25} Second, not all studies examined all outcomes, leading to low numbers available for the analyses. Consequently, robust statistical assessments for less-studied events like birthweight <2,500 g, chorioamnionitis, and neonatal sepsis were hindered. Thirdly, analyses of rare but clinically important outcomes may lack statistical power, as seen in reductions in spontaneous abortion or stillbirth (65% reduction) and delivery before the 34^th week of gestation (50% reduction) in the mono-infection group, which were not statistically significant. Nonsignificant results do not necessarily indicate the absence of an association but may reflect limited sample size, low statistical power, or the presence of confounding variables. Fourth, due to the limited number of studies available for several comparisons, we were unable to perform further stratified analyses by study design or geographic region, as such subgrouping would have compromised the statistical power and interpretability of the findings. Fifth, heterogeneity among studies may undermine the generalizability of conclusions, particularly in less-studied scenarios. Sixth, diagnostic criteria varied across primary studies. For Ureaplasma spp. and Mycoplasma hominis, diagnosis relied on PCR or culture positivity rather than validated quantitative thresholds; since these organisms are common commensals, PCR detection does not confirm clinically significant infection, and no study reported bacterial load or cycle-threshold values to distinguish true infection from colonization. This diagnostic ambiguity, inherent to the primary literature, may have caused non-differential misclassification affecting our pooled estimates for these organisms.

The findings are particularly relevant for obstetricians during antenatal counseling. Since women with one bacterial reproductive tract infection commonly have others, ^{Reference Borgogna, Shardell and Yeoman26} early diagnosis and investigation of additional infections is essential. If a single infection can harm both the mother and newborn, co-infections are associated with higher rates of adverse outcomes, though further research is needed to confirm causality.

Correlation with existing literature

Gonorrhea infection has been associated with preterm birth, premature rupture of membranes, perinatal mortality, and low birthweight ^{Reference Vallely, Egli-Gany and Wand4} and Chlamydia mono-infection with fetal death and preterm birth. ^{Reference Adachi, Nielsen-Saines and Klausner27} Given the frequency of bacterial co-infections, ^{Reference Adachi, Klausner and Xu3} systematically review which pregnancy outcomes are most affected by the number of non-viral infections was essential. To our knowledge, this is the most comprehensive systematic review and meta-analysis comparing non-viral mono-infections and co-infections during pregnancy, and the only one considering both maternal and neonatal outcomes. Previous studies have examined the impact of co-infections, in general, during gestation. It has been reported that pregnancies complicated by HIV have a higher risk of adverse outcomes, such as stillbirth and HIV vertical transmission, when co-infected with syphilis or tuberculosis. ^{Reference Yang, Cambou and Segura28–Reference Yilma, Bailey, Karakousis and Karanika30} There are also indications that ToRCH (toxoplasmosis, rubella, cytomegalovirus, and herpes simplex virus) co-infections may be associated with a higher risk of spontaneous abortion compared to single infections. ^{Reference Rasti, Ghasemi, Abdoli, Piroozmand, Mousavi and Fakhrie-Kashan31}

Beyond non-viral co-infections, interactions between viral and bacterial reproductive tract pathogens are well documented. The “epidemiological synergy” between HIV and bacterial STIs such as chlamydia, gonorrhea, and syphilis is well established: bacterial-induced genital inflammation increases both HIV shedding and acquisition risk, and treating the bacterial infection reduces this risk. ^{Reference Fleming and Wasserheit32} Notably, one of our nine included studies was conducted exclusively in HIV-infected pregnant women evaluating chlamydia/gonorrhea mono- and co-infection. ^{Reference Adachi, Klausner and Xu3} Together with the HIV-syphilis, HIV-tuberculosis, and ToRCH data discussed above, ^{Reference Yang, Cambou and Segura28–Reference Rasti, Ghasemi, Abdoli, Piroozmand, Mousavi and Fakhrie-Kashan31} these findings support the biological plausibility that co-infections, whether non-viral or viral-bacterial, may compound adverse outcomes through shared mechanisms of mucosal inflammation and impaired local immunity.

Congenital syphilis is a well-established risk factor for fetal and neonatal death, preterm birth, low birthweight, and extended hospital stays. ^{Reference Chen, Klausner and Stafford33,Reference Rocha, Araújo, Barros, Américo and Silva Júnior34} Given the rising number of syphilis cases among women of reproductive age, ^{Reference Rochel-Perez, Martin-Dorantes and Mendez-Dominguez35–Reference Hengel, McManus and Monaghan37} our review underscores the need for further studies that examine co-infections of syphilis with other bacterial infections.

The findings of this study, along with previous research, reinforce the importance of screening for other infectious diseases once one is diagnosed during pregnancy, whether bacterial, viral, or parasitic.

Recent research ^{Reference DePaoli Taylor, Hill and Perez-Patron38} has demonstrated that Chlamydia may be associated with gestational hypertension, preeclampsia with severe features, and superimposed preeclampsia. In contrast, gonorrhea was not found to be associated with hypertensive disorders of pregnancy. Nevertheless, although gonorrhea may not directly elevate the risk of hypertensive disorders, it could still contribute to the development of these conditions in certain cases. More studies are needed to clarify this relationship.

In the latest Sexually Transmitted Infections Treatment Guidelines, ^{Reference Workowski, Bachmann and Chan39} the Centers for Disease Control and Prevention (CDC) emphasize the importance of screening for bacterial sexually transmitted infections in pregnant women. Syphilis screening is recommended at the first prenatal visit, with additional testing in the third trimester and at delivery for women at high risk. Timely detection and treatment are essential to prevent congenital syphilis, which has significantly increased in the United States. Similarly, screening for chlamydia and gonorrhea is advised for all pregnant women under 25 years old and those at increased risk. Women with ongoing risk factors should be retested later in pregnancy to avoid maternal and neonatal complications. Immediate treatment of positive cases is crucial, followed by a test of cure to ensure eradication. Although evidence does not support routine screening for asymptomatic bacterial vaginosis in pregnant women, those showing symptoms should be evaluated and treated accordingly. The focus is on individualized care, early detection, prompt treatment to minimize adverse outcomes such as preterm delivery and neonatal infections.