Severe maternal morbidity (SMM) refers to “potentially life-threatening conditions during pregnancy, childbirth or after pregnancy, from which maternal near miss (MNM) cases would emerge” (1, 2). The standard definition and internationally accepted identification criteria for SMM by the World Health Organization (WHO) Working Group on Maternal Mortality Morbidity Classifications (1, 2) incorporate clinical disorders (hemorrhagic, hypertensive, and other systemic disorders) and severe management indicators to assess the severity of SMM and to strengthen the ability of health care professionals to detect SMM (1).

However, maternal mortality and SMM remain major public health problems for global healthcare systems despite substantial progress (3). Between 1990 and 2015, the global maternal mortality ratio (MMR) declined by 44% but failed to meet the 75% reduction set by Millennium Development Goal 5. Sustainable Development Goal 3 aims to ensure healthy living, promote well-being for people of all ages, and reduce global MMR to <70 per 100,000 live births by 2030. The average reduction of 7.5% each year between 2016 and 2030 is more than three times the 2.3% annual rate of decline observed globally between 1990 and 2015.

Prior to 2011, the literature varied in its definitions and criteria used to classify SMM and MNM cases. In a review, the prevalence of SMM based on various disease-specific, management-based, and organ system dysfunction-based criteria ranged from 0.04 to 15% (4). Meanwhile, the prevalence according to organ-based dysfunction using the Mantel criteria and emergency hysterectomy was 0.42 and 0.039%, respectively. With a 98.3% I² value for the Mantel-based criteria and 95.5% for the emergency hysterectomy criteria, the heterogeneity among studies was large and significant (4).

While maternal mortality has been commonly used as a benchmark for maternal health status, there is evidence that it is just the “tip of the iceberg” for adverse maternal outcomes (5, 6). The SMM literature has revealed several contributing factors. Employment status (7), low household income (8), history of abortion (8), multiple births (9), and inadequate antenatal care (8) were documented as contributing factors for SMM. However, mixed findings were reported for the factors of age (10–12), race (9, 13), educational level (8, 13), co-existing medical conditions (14), parity (15), gestational period (10), mode of delivery (12, 16), previous cesarean section (14), and pre-pregnancy body mass index (9, 14).

Determining the pooled prevalence of SMM at a global level gives a better indication of its severity than discrete primary studies. The identification of factors associated with SMM allows a clearer understanding of the issue and serves as a basis for appropriate preventive strategies to be established. This pertains to primary prevention at the institutional, provider, and client levels, such as screening or preventive that could potentially avoid death or severe morbidity from a disorder. Therefore, to inform clinical policy and enhance patient care, planning beyond maternal mortality and direct, focused evaluation of the strength of each risk factor's association with SMM is required. This systematic review aims to ascertain the prevalence and risk factors of SMM based on the WHO standard definition and criteria (1). The evidence, effect estimates, and strength of statistical association between SMM and its risk factors will be summarized in this review.

In our analysis, the worldwide prevalence of SMM and MNM was 2.45 and 1.68%, respectively. The risk factors of history of cesarean section, advanced maternal age, multiple pregnancies, co-existing medical conditions, and cesarean birth were associated with SMM, while the sole reported risk factor for MNM was a history of cesarean section.

Older maternal age has been associated with a greater likelihood of having pre-existing medical conditions, a higher risk of obstetric complications, maternal morbidity, and an increased risk of progression from SMM to death (42). In our review, it can be concluded that advanced maternal age increases the risk of SMM. Contrary to the results of other reviews, we found that maternal morbidity was higher in mothers at both ends of the age spectrum, such as those of age >35 years and adolescents (43).

According to a review (43), national estimates from vital registration in the United States suggest that adolescents have a lower MMR than women of age >20 years. The same review also reported statistics from the United Kingdom showing that the MMR for women of age under 20 years is slightly higher than that of women of age 20–24 years but lower than that of women of age >25 years. Conversely, data from Australia suggest that the MMR for women of age under 20 years is higher than those of all other age groups apart from women of age >40 years. Evidence from both developing and developed countries also suggests that risks are greater for younger adolescents, with girls of age 15 years or younger having higher mortality rates than older adolescents. In most countries, adolescent births are concentrated among poorer, less educated women, which further compounds their disadvantaged situation by disrupting school attendance and limiting future livelihood opportunities.

In terms of physiological risk, the evidence is somewhat conflicting. Still, some evidence suggests that young women are at increased risk of several direct causes of maternal mortality, such as eclampsia, and indirect causes, such as obstructed labor. A study in Latin America also showed increased risks of hemorrhage and sepsis in young women, particularly those under 16 years of age (43).

According to a report by the Centers for Disease Control and Prevention, the MMR in the United States increased gradually with increasing maternal age and particularly in women older than 35 years, specifically, a 3.5-fold increase in MMR. This increased rate of maternal mortality is mostly attributed to comorbidities and coexisting medical conditions, such as metabolic syndrome, past and current cancer, cardiovascular, renal, and autoimmune diseases, which are more prevalent among older pregnant women. For instance, hypertension and Type II diabetes are more common in older women, and women older than 35 years have a 2- to 4-fold higher risk of having hypertension compared to women aged 30–34 years (44).

Women with a prior cesarean delivery had almost two times the risk (CI: 1.93, 2.23) of SMM compared to those without a prior cesarean delivery. Prior cesarean delivery is known to be associated with preeclampsia, placenta previa, placenta accreta, placental abruption, uterine rupture, postpartum infection, transfusion, and admission to the intensive care unit (14). The risks of placenta previa and morbidly adherent placenta in subsequent pregnancies increased markedly with an increasing number of prior cesarean sections. Compared with those without a history of cesarean section, pregnant women with a history of cesarean section were significantly more likely to experience uterine rupture, morbidly adherent placenta, MNM, severe maternal outcomes, and placenta previa. Uterine rupture during pregnancy and delivery is one of the most devastating obstetric complications, as it frequently results in life-threatening maternal and fetal compromises (45).

According to the 2018 annual report by the National Perinatal Epidemiology Center of Ireland, 45.4% of women who experienced an SMM in 2016 were nulliparous (46). The SMM rate for multiparous women was 6.46 per 1,000, 32% higher than that of primiparous women. However, nulliparous women had the highest SMM of 7.66 per 1,000 births, 56% higher than primiparous women at 4.91 per 1,000 births.

A French study found that the risk for SMM was significantly higher for unplanned cesarean deliveries at all maternal ages. In contrast, the planned delivery mode analysis showed an increased risk of SMM associated with planned cesarean delivery compared with planned vaginal delivery in only women aged 35 years and older (42).

According to a secondary analysis by the WHO Multicountry Survey on Maternal and Newborn Health (WHOMCS), a cross-sectional study performed in 29 countries, it was found that twin pregnancies had a higher risk of contributing to SMM and mortality in all regions (47). Using the new WHO diagnostic criteria for severe maternal conditions, another recent WHOMCS study identified that twin pregnancies doubled the risk of SMM, tripled the risk of MNM, and increased the risk of maternal death 4-fold when compared to singleton pregnancies. The current review found a higher occurrence of preterm deliveries in SMM, which correlates with the underlying etiology of the women's medical conditions.

According to the WHO, maternal deaths have been described as the tip of the iceberg and maternal morbidity as the base (5). For every woman who dies from pregnancy-related reasons, another 20 to 30 women suffer from acute or chronic morbidity, frequently with long-term consequences that impair their ability to function normally. Women's physical, mental, and sexual health, as well as their ability to perform in particular areas (e.g., cognition, mobility, and social involvement), body image, and socioeconomic position, are all affected by these consequences. Maternal morbidity, like maternal mortality, is expected to be the highest in low- and middle-income nations, particularly among the poorest women. There are no proper standardized results on the implications of SMM. However, it is well-known that it can lead to short- and long-term consequences, which are preventable.

The studies in the current review underwent quality assessment, and only good-quality studies with a low risk of bias were included. There were only a few studies that compared women with and without maternal complications, which did not allow us to identify population risk factors. The heterogeneity in the outcomes ranged from low to high, possibly due to the participants' sociodemographic characteristics. However, subgroup analysis was not possible because of the limited number of studies that had substantial heterogeneity. We involved only three databases for searching. However, MEDLINE is the most extensive database for social and medical sciences and the inclusion of 24 papers is strong enough to conclude the outcome. Moreover, we have displayed the cumulative effect estimates in forest plots that consolidate the findings.

Notably, identifying the risk factors for severe maternal morbidity and maternal near miss is pertinent to prevent maternal death. Quantifying the strength of association of the risk factors and surveillance of the presence of these factors at primary and tertiary care levels helps to identify the interventions required that can be performed during antenatal care and childbirth. Initiatives such as educational programs, campaigns, and early detection of risk factors contributing to SMM and MNM are recommended. Proper follow-up is important to monitor the progression of maternal health during the antenatal and postnatal periods. We recommend additional studies comparing women with and without SMM and MNM using the WHO definition and criteria.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

NHNH, MNN, and ISB: conceptualization, methodology, and visualization. MNN and NHNH: validation, resources, and data curation. MNN and HRMK: formal analysis and writing of original draft preparation. HRMK: investigation. NHNH, MNN, ISB, and HRMK: writing of review and editing. NHNH: project administration. All authors have read and agreed to the published version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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