1. This retrospective cohort of singleton pregnancies found that severe maternal morbidity (SMM) was significantly higher in women who had been exposed to long-term heat during pregnancy or heat waves during the last gestational week.
2. The rate of SMM was also significantly higher in the third trimester of pregnancy, specifically. This implies a potential role for acknowledging a changing climate as a risk factor for SMM.
Evidence Rating Level: 2 (Good)
Study Rundown: Severe maternal morbidity (SMM) refers to severe and unexpected conditions that arise during labour and delivery, and the body of research surrounding this topic is expansive and well-established. While substance-related, socioeconomic, race and ethnicity-related, and other risk factors for pregnant women have been identified and elucidated in the literature, the United States continues to see an increase in the rates of SMM.
This retrospective cohort study aimed to assess an environmental risk factor that remains poorly studied concerning maternal morbidity: heat exposure, particularly in a changing climate. The current study employed data from a pregnancy cohort in Southern California to evaluate the relative risk of maternal morbidity as it relates to heat exposure.
Researchers found that more heat days during pregnancy were associated with increased risk for SMM. This was a dose-dependent effect, with mothers exposed to more severe heat/heat waves during pregnancy (and specifically in the third trimester) experiencing significantly higher rates of SMM. These results indicate that the third trimester specifically may be a more sensitive period for heat exposure.
In-Depth [retrospective cohort]: The study population consisted of patients with singleton pregnancies from a pregnancy cohort in Kaiser Permanente Southern California, between 2008 and 2018. Pregnancies were excluded from analysis if there was not enough temperature data available during a pregnancy. The primary outcome was SMM20 (SMM without requirement for transfusion of blood products) during delivery. One of the secondary outcomes studied was SMMcardio (the cardiovascular subset of conditions of maternal morbidity). A total of 403,602 singleton pregnancies were analyzed, with the racial distribution being 7.8% African-American, 12.6% Asian, 50.8% Hispanic, 26.6% non-Hispanic White, and 2.6% other races. Of these pregnancies, 3446 were identified as having experienced SMM20, and 684 experienced SMMcardio. When adjusting for several extraneous factors (including race, maternal age, socioeconomic factors, season of conception, and year of delivery), there was found to be a significantly higher risk of SMM20 and SMMcardio in pregnancies with long-term heat exposure during the third trimester, as well as over the whole pregnancy. The strength of these associations was mediated by the severity of heat exposure, with severe exposures over the third trimester associated with a 28% increase in SMM20 (OR, 1.28; 95% CI, 1.17-1.41; p < .05), and 51% increase in SMMcardio (OR, 1.51; 95% CI, 1.22-1.87; p < .001). The most severe heat exposure over the course of a whole pregnancy was found to increase the risk of SMM20 by 27% (OR, 1.27; 95% CI, 1.17-1.37; p < .05) and SMMcardio by 48% (OR, 1.48; 95% CI, 1.25-1.76; p < .05). A similar dose-dependent increase in SMM20 was found for mothers who endured exposure to short-term heatwaves. Mothers with exposure to temperatures at or above the 95th percentile for four or more days in the last gestational week were 139% more likely to experience SMM20 (OR, 2.39; 95% CI, 1.62-3.54; P < .001). Additionally, a significant interaction between a mother’s education level and the season of conception was observed. That is, mothers with lower education levels who were exposed to long-term heat experienced significantly more SMM20 than mothers of higher education levels (less than college education OR, 1.43; 95% CI, 1.26-1.63; p = .02), and mothers who conceived between November and April suffered more SMM20 than those who conceived between May and October when exposed to the same amounts of heat (November to April OR, 1.37; 95% CI, 1.24-1.53; P < .001). Both long-term and short-term heat exposure were associated with increases in maternal morbidity related to sepsis. Overall, this study found that climate and heat exposure, particularly during the final trimester, could be yet another risk factor that providers and mothers keep in mind to prevent SMM. An important limitation to note is that the proportion of time women spent indoors vs outdoors on these higher heat days was not available for analysis. Further studies should examine maternal morbidity beyond the delivery period to analyze long-term outcomes, and replications of this study should be employed in different geographical regions to determine the generalizability of results.
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