Severe maternal morbidity in twin pregnancies: the impact of body mass index and gestational weight gain

Introduction

Obesity is associated with numerous pregnancy complications, including life-threatening adverse outcomes for the mother, collectively referred to as severe maternal morbidity (SMM) [1]. The risk of SMM increases progressively with each sequential body mass index (BMI) classification group, demonstrating the critical impact of maternal weight on pregnancy outcomes [2], [3], [4]. While most prior studies examining the relationship between BMI and SMM have focused on singleton pregnancies, twin pregnancies inherently carry an increased risk of SMM and maternal mortality compared to singleton gestations [5], 6]. In addition to BMI, gestational weight gain (GWG) is an important factor influencing maternal and neonatal outcomes in twin pregnancy [7], [8], [9]. Excessive GWG may exacerbate risks associated with preeclampsia, gestational diabetes, and preterm delivery, particularly in twin pregnancies. Prior investigations into twin pregnancies have largely focused on common pregnancy outcomes rather than SMM, likely due to the rarity of these severe events. The objective of this study was to determine whether pre-pregnancy BMI and GWG, individually or in combination, are associated with SMM in twin gestations.

Materials and methods

This retrospective cohort study evaluated all deliveries of twin gestations at≥20 weeks of gestational age from seven hospitals within a large academic health system in New York between 2019 and 2023. Patients with a pre-pregnancy BMI classified as underweight, as well as those with missing BMI or GWG data, were excluded. Clinical data were obtained from the inpatient electronic health record system (Sunrise Clinical Manager, Allscripts Corp., Chicago, IL). Sociodemographic information, medical comorbidities, and obstetrical history was collected. Maternal comorbidities were identified by International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10) codes and clinical documentation. Pre-pregnancy height and weight, and GWG at last visit before delivery were obtained from the prenatal record; when unavailable, self-reported data collected at the time of delivery hospitalization was used.

The primary exposures were maternal pre-pregnancy BMI group and excessive GWG. BMI group was classified as follows: normal weight (BMI 18.5–24.9 kg/m²), overweight (BMI 25–29.9 kg/m²), class 1 obesity (BMI 30–34.9 kg/m²), class 2 obesity (BMI 35–39.9 kg/m²), and class 3 obesity (BMI≥40 kg/m²). Excessive GWG (yes/no) was defined based on the Institute of Medicine (IOM) guidelines for twin pregnancies as exceeding the recommended upper limit of total weight gain for a given pre-pregnancy BMI category: more than 54 pounds for patients with a normal BMI, more than 50 pounds for overweight patients, and more than 42 pounds for obese patients [10]. Covariate factors assessed were race and ethnicity group, and a modified obstetric comorbidity index (OB-CMI) score. Race and ethnicity were self-selected from prespecified categories. The OB-CMI score, calculated at the time of admission for delivery, was modified for our analysis to exclude the scoring components for BMI>40 kg/m² (score of 2) and BMI>50 kg/m² (score of 3), thus allowing for separate analysis of the BMI variable. The OB-CMI screening tool was developed to predict SMM risk based on the number and type of maternal comorbidities and pregnancy complications [11], 12]. The weighted scoring system assigns points to various conditions as follows: preeclampsia with severe features (5), preeclampsia/gestational/chronic hypertension (2), congestive heart failure (5), pulmonary hypertension (4), ischemic heart disease/cardiac arrhythmia (3), congenital heart and/or valvular disease (4), multiple gestation (2), intrauterine fetal demise (2), placenta previa/suspected accreta/abruption (4), previous cesarean delivery/myomectomy (1), autoimmune disease/lupus (2), HIV/AIDS (2), sickle cell disease/bleeding disorder/coagulopathy/anticoagulation (3), epilepsy/cerebrovascular accident/neuromuscular disorder (2), chronic renal disease (1), asthma (1), diabetes on insulin (1), maternal age>44 (3), maternal age 40–44 (2), maternal age 35–39 (1), substance use disorder (2), and alcohol abuse (1).

The primary outcome was SMM that occurred during the delivery hospitalization; this was defined using the United States Centers for Disease Control and Prevention (CDC) criteria which includes 21 indicators identified from ICD-10 diagnosis and procedural codes [13]. The rate of SMM was compared between BMI groups.

Categorical variables are expressed as frequency and percentage and continuous variables as mean and standard deviation. Chi-square or Fishers exact tests were used to evaluate frequency distributions of categorical variables. Kruskal Wallis rank sum tests or analysis of variance (ANOVA) tests were used to compare ranks or means respectively of continuous variables. Multivariable logistic regression was used to model the probability of SMM as a function of BMI group, adjusting for excessive GWG (yes/no), race and ethnicity group, and modified OB-CMI score. Odds ratios (OR) and 95 % confidence intervals (CI) are presented. The sample size was based on availability of data and was not established using any formal statistical power calculation. Analyses were conducted using R 4.3.1. The Northwell Health Institutional Review Board approved this study as minimal-risk research using data collected for routine clinical practice and waived the requirement for informed consent.

Results

A total of 1,976 twin gestations met inclusion criteria. Non-Hispanic White patients constituted the largest race and ethnicity group (43.1 %), followed by Hispanic (16.9 %), Non-Hispanic Black (15.8 %), and Asian or Pacific Islander (9.2 %). Patient sociodemographic and clinical characteristics are shown in Table 1. Maternal comorbidity burden, as quantified by the modified OB-CMI score, increased with successive BMI group. A modified OB-CMI score of four or more was observed in 45.5 % of normal weight patients compared to 49.5 % of patients with class 3 obesity. Patients with pre-pregnancy obesity were more likely to exceed GWG recommendations than patients with a normal pre-pregnancy BMI (24.2 vs. 14.4 %, respectively).

In this twin cohort, the overall SMM rate was 14.0 % (n=276) with 356 independent SMM events. There were no cases of maternal mortality. The most common SMM event was blood product transfusion (n=224). Table 1 shows the five most frequent SMM indicators stratified by BMI group. Results of multivariable logistic regression modeling are shown in Table 2. When normal weight patients were compared to those in higher BMI categories, no significant differences in SMM were observed, either before or after adjustment for covariates. Similarly, no significant differences in SMM were found when patients with appropriate GWG were compared to those who exceeded GWG recommendations, both before and after covariate adjustment. SMM was experienced more frequently in racial and ethnic minority groups, and those with higher modified OB-CMI scores. Compared to non-Hispanic White patients, those who self-identified as non-Hispanic Black, Hispanic, Asian or Pacific Islander, or multiracial/other were at increased risk.

Discussion

Neither pre-pregnancy BMI nor GWG was associated with SMM in our twin cohort, both before or after adjustment for sociodemographic and clinical covariates. Thus, the degree or severity of obesity and weight gain does not contribute to the already elevated baseline risk of SMM in this high-risk patient population.

After reviewing the literature, we identified key studies addressing the relationship between GWG, BMI, and maternal and neonatal outcomes in twin pregnancies. However, no prior studies specifically explored their association with SMM, likely due to the rarity of these outcomes and the large sample sizes needed for such stratified analysis. Prior studies evaluating the effect of obesity on pregnancy outcomes have found that preeclampsia, gestational diabetes, and cesarean delivery rates increased with higher maternal BMI in both singleton and twin gestations, but the association was weaker in twins [14], 15]. Over half of twin pregnancies are affected by inappropriate GWG [16]. Schubert et al. found that excessive GWG in twin pregnancies was associated with hypertensive disorders and cesarean delivery [9]. Bodnar et al. similarly reported that excessive GWG negatively impacted fetal growth and maternal outcomes in twin pregnancy [7]. Lin et al. stratified optimal GWG ranges by BMI, noting that individuals with obesity required lower GWG than those with normal or low BMI to reduce adverse outcomes [17]. Our study builds on this evidence by focusing on SMM in twin pregnancies. While obesity’s impact on singleton pregnancy outcomes is well-documented, research on twin pregnancies is limited. This study adds to the literature by examining how BMI and GWG contribute to SMM risk in this high-risk population. Our findings should be confirmed in other geographic regions with different patient populations and practice patterns.

This study has several strengths. The patient and obstetrical provider populations were diverse. Multiple facilities within a large health system were included. Utilization of data from a shared electronic health record system across all hospital sites minimized data extraction errors and misclassification. Our study also has limitations, including its retrospective design and use of administrative data. There is a risk of erroneous or incomplete billing data and inconsistent coding practices. Since this study used billing codes to identify SMM cases, it is unknown whether the event(s) occurred pre- or post-delivery. Given the overall rarity of SMM and relatively small number of patients within each BMI group, we cannot exclude the possibility of a type II error.

In summary, while SMM occurred in more than one in eight twin pregnancies in our cohort, pre-pregnancy BMI and GWG are not associated with higher odds of SMM. These findings are important for patient counseling and clinical risk assessment.