During the COVID-19 pandemic, the prevention and control measures, or non-pharmaceutical interventions (NPIs), blocked the transmission of many respiratory pathogens, including M. pneumoniae (Meyer Sauteur et al., 2022). Therefore, during the period of NPIs, the detection rate of M. pneumoniae was extremely low, until the first half of 2023, when there was an observable resurgence, indicating a new outbreak of M. pneumoniae, particularly in Asia (Meyer Sauteur et al., 2024).

It is estimated that 97% of the population in China had been infected with SARS-CoV-2 before January 2023 (Goldberg et al., 2023). Growing evidence indicates that SARS-CoV-2 infection can induce a prolonged phase of immune suppression and inflammatory injury, characterized by reduced counts of natural killer (NK) cells, lymphocytes, and monocytes, alongside elevated levels of inflammatory cytokines (Peluso et al., 2021; Ryan et al., 2022). Notably, immune dysregulation and inflammatory damage are recognized as key pathogenic mechanisms in severe M. pneumoniae pneumonia (SMPP) (Zhang et al., 2024; Yang et al., 2024). This suggests that the COVID-19-induced immunological impairment may potentially increase the risk of M. pneumoniae-infected patients progressing to severe disease. Patients with coinfection (COVID-19 and M. pneumoniae) have higher mortality compared with patients with just COVID-19 disease (Amin et al., 2021). This raises our interest in whether the prognosis of M. pneumoniae infection would become worse in the post-COVID-19 era (the period after January 2023 (Li et al., 2024)).

Pneumonia in pregnancy also results in low-birth-weight neonates in 33.9% of cases compared with 13.6% of controls (Goodnight and Soper, 2005). Tang et al. observed high incidences of adverse fetal outcomes in patients with severe pneumonia (Tang et al., 2018). Similarly, Chen et al. found that women with pneumonia during pregnancy had significantly higher risk of low birth weight, preterm birth, small for gestational age (SGA), low Apgar scores, cesarean section (CS), and preeclampsia/eclampsia, compared to without pneumonia (Chen et al., 2012). In the pregnant patient, pneumonia is the most frequent cause of fatal non-obstetric infection and M. pneumoniae is a common organism identified (Lim et al., 2001). From a biological perspective, a prior SARS-CoV-2 infection could potentially modify the maternal immune response to a subsequent M. pneumoniae infection, possibly leading to a more pronounced inflammatory response or an altered clinical course. Such a shift in host-pathogen interaction might result in maternal-fetal outcomes that differ from those observed in the pre-pandemic era, when the immune system had not been primed by SARS-CoV-2. Nevertheless, it remains unclear whether infection with M. pneumoniae affects adverse pregnancy outcomes in pregnant women in the post-COVID-19 era.

This paper aims to investigate the association between infection with M. pneumoniae and adverse pregnancy outcomes in pregnant women in the post-COVID-19 era, as well as to report the clinical manifestations of pneumonia caused by M. pneumoniae.

The COVID-19 pandemic has significantly diminished the populace’s immunity (Wu et al., 2024), which may result in a poorer prognosis when pneumonia occurs post-COVID-19 era compared to pre-COVID-19 era. This study focused on the outbreak of M. pneumoniae infections following the COVID-19 pandemic and provided some valuable insights into the potential impact of M. pneumoniae infection on adverse pregnancy outcomes among pregnant women, based on data from Guangzhou, China.

We first analyzed the clinical manifestations and laboratory findings of hospitalized pregnant women with M. pneumoniae infection after the COVID-19 pandemic (2023). Upper respiratory tract infection (URI) was the most common clinical presentation of M. pneumoniae infection, with the majority of pregnant patients exhibiting fever and cough as predominant symptoms (Layani-Milon et al., 1999). Respiratory manifestations were the most prominent, often accompanied by pronounced pharyngodynia, nasal congestion, headache, and asthenia, indicative of systemic toxicity. In contrast, extrapulmonary manifestations were uncommon and generally mild. These findings were consistent with the Expert Consensus on the Diagnosis and Treatment of M. pneumoniae Pneumonia in Adults, developed by the Infection Group of the Respiratory Society of the Chinese Medical Association. These clinical features were also comparable to those reported in adult M. pneumoniae infections both before and after the COVID-19 pandemic (Centers for Disease Control and Prevention (CDC), 2013; Li et al., 2024). Laboratory findings revealed that pregnant women with M. pneumoniae infection had white blood cell (WBC) counts approaching the upper limit of the normal adult range, while lymphocyte counts were slightly below the normal threshold, aligning with previous reports (Li et al., 2024). M. pneumoniae infection has been associated with a broad decline in immune cell populations, including lymphocytes, CD3⁺ T cells, CD4⁺ T cells, CD8⁺ T cells, and B cells, suggesting potential immunological alterations. Regarding inflammatory markers, levels of C-reactive protein (CRP), procalcitonin (PCT), and interleukin-6 (IL-6) were significantly elevated, indicating a robust inflammatory response. This trend is consistent with the findings of Li et al (Li et al., 2024), who reported similar elevations in CRP, PCT, and IL-6 in pediatric M. pneumoniae infections in Guangzhou following the COVID-19 pandemic, with these markers correlating with infection severity. Moreover, our results demonstrated no significant impairment of hepatic or renal function in pregnant women with M. pneumoniae infection. However, the potential impact of prolonged or severe M. pneumoniae infection on hepatic and renal function requires further investigation.

Following propensity score analysis, our findings suggest that in the post-COVID-19 era, evidence remains insufficient to conclude that M. pneumoniae infection increases the risk of adverse pregnancy outcomes in the overall population. However, some subgroups showed statistical significance. This contrast may be attributed to the study population being insufficiently specific to capture potential high-risk groups (e.g., the AMA population), thereby limiting our ability to detect statistical significance.

A possible reason for the lack of observed significance in the overall population is the small sample size of the cohort, with only 49 participants in the exposure group, which may have limited our ability to detect the differences (Sullivan and Feinn, 2012). Therefore, a post-hoc power analysis was conducted to evaluate the statistical power. The results showed that for the logistic regression analysis, a total sample of 186 participants (49 exposure group vs. 137 control group) achieves 54% power (implying a 46% Type II error rate) at a two-tailed 0.05 significance level to detect an OR of 2.00, assuming a baseline risk of 0.4 (Supplementary Figure S13). A problem accompanying the small sample size was the low number of events (e.g., no participants in the M. pneumoniae group experienced postpartum hemorrhage), which can lead to considerable upward or downward bias in estimates obtained from standard ML. Given this unavoidable limitation, Firth’s penalized logistic regression was used to correct small sample bias, thereby preventing more extreme and implausible estimates. This issue of small sample size was particularly pronounced in the subgroup analyses. For instance, the AMA subgroup included only 29 participants.

Subgroup analyses for adverse maternal outcomes revealed two findings: M. pneumoniae infection significantly increased the risk of adverse maternal events and CS in primiparous women, while women without a history of abortion also exhibited a higher CS rate. These findings would be of substantial importance if the associations are confirmed in adequately powered studies. Prior to the COVID-19 pandemic, multiple clinical studies on pneumonia in pregnant women identified CS as a major adverse pregnancy outcome (Tang et al., 2018; Chen et al., 2012; Romanyuk et al., 2011). Our findings in the post-pandemic era are consistent with these reports, particularly in the subgroup of pregnant women with infection with M. pneumoniae. This increased CS rate may be attributed to the elevation of inflammatory cytokines following pathogen infection, which can impair uterine contractility and cervical dilation, subsequently increasing the likelihood of CS. Studies have demonstrated that the activation of inflammatory transcription factors such as NF-κB may directly upregulate genes associated with uterine contractions, thereby promoting the contractility of uterine smooth muscle (Mendelson, 2009). Furthermore, monocytes recruited to the cervix and myometrium, upon activation, release pro-inflammatory cytokines including IL-1β, TNF-α, and IL-6, which participate in cervical ripening, membrane rupture, and the initiation of labor (Obeagu, 2025). M. pneumoniae infection has been shown to significantly elevate the levels of cytokines such as IL-6, IL-8, and TNF-α (Zhao et al., 2019). The findings from Li et al (Li et al., 2024). indirectly lend support to the potential importance of this mechanism. Notably, within the amniotic fluid environment, IL-6 and IL-8 levels have been established as robust predictors of adverse fetal outcomes (McCartney et al., 2021). The impact of infection with M. pneumoniae on adverse maternal outcomes was more pronounced in primiparous women, potentially due to both psychological and physiological factors. Psychologically, the absence of prior childbirth experience may contribute to heightened fear and anxiety, which could affect stress responses and lead to labor abnormalities. Physiologically, the uterus and birth canal of primiparous women may exhibit increased susceptibility to infection, further predisposing them to adverse outcomes. Similarly, pregnant women without a history of abortion may have a relatively fragile reproductive system, lacking prior immunological adaptation to pregnancy-related inflammatory challenges, which could make them more vulnerable to the adverse effects of M. pneumoniae.

In terms of adverse neonatal outcomes, subgroup analyses revealed that, in AMA, the incidence of preterm birth, fetal distress, and other infections was significantly higher in the M. pneumoniae group. Moreover, neonates in this subgroup exhibited a marked tendency toward reduced birth length, weight, and head circumference. Although these findings from exploratory subgroup analyses require further validation given the limited sample size, several mechanisms may explain these observations. First, AMA is inherently associated with multiple obstetric risk factors, predisposing to adverse pregnancy outcomes, which is consistent with the meta-analysis findings of Lean et al (Lean et al., 2017a). A widely accepted explanation is that accelerated placental senescence in AMA pregnancies leads to altered nutrient transport and vascular function, thereby compromising the intrauterine environment for fetal growth. Similar characteristics have been observed in aged murine models (Lean et al., 2017b). Second, the genetic integrity of oocytes declines with maternal aging (Cimadomo et al., 2018), and while the relationship between this phenomenon and adverse pregnancy outcomes remains to be fully elucidated, it warrants further investigation. Additionally, in pregnancies at AMA complicated by infection with M. pneumoniae, the infection and its associated inflammatory response constitute a major risk factor for preterm birth (Cappelletti et al., 2016). This heightened inflammatory burden may impose additional stress on the placenta and uterus, increasing the risk of fetal distress and further restricting fetal growth, ultimately leading to reductions in neonatal birth length, weight, and head circumference. These findings align with clinical studies conducted before the COVID-19 pandemic on pneumonia-complicated pregnancies (Chen et al., 2012).

These findings should be interpreted in the context of specific limitations. First, the single-center design of this study may restrict the generalizability of our findings. Additionally, statistical power was insufficient, as post-hoc analysis indicated 54% power overall, implying a 46% Type II error rate. Similarly, the exploratory subgroup analyses were constrained by small sample sizes (particularly in the AMA subgroup), complete separation, and the potential for spurious findings due to multiple comparisons across outcomes and subgroups. Consequently, the subgroup findings are hypothesis-generating and require validation in larger, adequately powered studies.

In the post-COVID-19 era, evidence remains insufficient to conclude that M. pneumoniae infection increases the risk of adverse pregnancy outcomes in the general pregnant population. However, our findings suggest potential risks specifically within subgroups of AMA and primiparous women. Despite limitations regarding sample size and the single-center design, it provides clinically relevant insights into patterns of M. pneumoniae infection in the post-COVID-19 era. These findings offer valuable guidance for clinical management and inform therapeutic decision-making following infection.