Elsevier

Environment International

Volume 111, February 2018, Pages 295-300
Environment International

Heat and pregnancy-related emergencies: Risk of placental abruption during hot weather

https://doi.org/10.1016/j.envint.2017.11.004Get rights and content

Highlights

  • We studied the association between ambient temperature and placental abruption.

  • Risk of placental abruption was associated heat, particularly for women at term.

  • Relationships were more prominent for pregnant women younger than 35 years old.

  • Associations were stronger for nulliparous and disadvantaged women.

Abstract

Introduction

Outdoor heat increases the risk of preterm birth and stillbirth, but the association with placental abruption has not been studied. Placental abruption is a medical emergency associated with major morbidity and mortality in pregnancy. We determined the relationship between ambient temperature and risk of placental abruption in warm seasons.

Material and methods

We performed a case-crossover analysis of 17,172 women whose pregnancies were complicated by placental abruption in Quebec, Canada from May to October 1989–2012. The main exposure measure was the maximum temperature reached during the week before abruption. We computed odds ratios (OR) and 95% confidence intervals (CI) for the association of temperature with placental abruption, adjusted for humidity and public holidays. We assessed whether associations were stronger preterm or at term, or varied with maternal age, parity, comorbidity and socioeconomic status.

Results

Compared with 15 °C, a maximum weekly temperature of 30 °C was associated with 1.07 times the odds of abruption (95% CI 0.99–1.16). When the timing of abruption was examined, the associations were significantly stronger at term (OR 1.12, 95% CI 1.02–1.24) than preterm (OR 0.96, 95% CI 0.83–1.10). Relationships were more prominent at term for women who were younger than 35 years old, nulliparous or socioeconomically disadvantaged, but did not vary with comorbidity. Associations were stronger within 1 and 5 days of abruption. Temperature was not associated with preterm abruption regardless of maternal characteristics.

Conclusions

Elevated temperatures in warm seasons may increase the risk of abruption in women whose pregnancies are near or at term. Pregnant women may be more sensitive to heat and should consider preventive measures such as air conditioning and hydration during hot weather.

Introduction

A growing number of studies suggest that environmental conditions can affect the risk of placental abruption (Mankita, 2012, Michikawa et al., 2017, Yackerson et al., 2007). Placental abruption is an obstetric emergency that in extreme cases can lead to severe maternal and fetal morbidity or mortality (Oyelese and Ananth, 2006, Tikkanen, 2011). Abruption occurs when the placenta separates prematurely from the wall of the uterus before delivery, causing hemorrhage (Oyelese and Ananth, 2006). Every 7–12 pregnancies per 1000 end in abruption in North America (Ananth et al., 2014), with data suggesting that rates may be increasing (Ananth et al., 2014, Broers et al., 2004, Oyelese and Ananth, 2006). Preventing placental abruption is difficult as its causes are not fully understood, and risk factors such as preeclampsia and substance abuse account for only a proportion of cases (Tikkanen, 2011). Recent studies suggest that exposure to air pollutants may increase the risk of abruption (Ibrahimou et al., 2017, Michikawa et al., 2017), and that unstable seasonal weather in semi-arid areas can contribute (Mankita, 2012, Yackerson et al., 2007). The possibility that elevated outdoor temperature could increase the risk of placental abruption has however not been investigated.

Elevated temperature has potential to trigger placental abruption through different routes. In normal individuals, outdoor heat stress results in diversion of blood flow to the skin surface to prevent increasing core body temperature (Wells and Cole, 2002). Pregnant women may however not be able to regulate temperature as efficiently due to the physiologic changes of gestation (Schifano et al., 2016). Heat may alter placental blood flow patterns, potentially reducing the integrity of the placenta and increasing the chance of abruption. Also, elevated temperatures have been shown to increase fetal heart rate and lead to uterine contractions (Vaha-Eskeli and Erkkola, 1991), both of which are associated with placental abruption (Tikkanen, 2011). Heat could further affect risk factors for abruption, such as preeclampsia and prolonged preterm premature rupture of membranes (Beltran et al., 2014, Yackerson et al., 2007). Related outcomes such as preterm birth, low birth weight, and stillbirth have been shown to be more frequent during high temperatures, thought to be due to the added strain that heat exerts on pregnant women (Auger et al., 2016a, Arroyo et al., 2016, Bruckner et al., 2014, Dadvand et al., 2011, Ha et al., 2017a, Ha et al., 2017b, Schifano et al., 2016). The possibility of an association with placental abruption however has not been studied, despite similarities in biologic pathways of abruption, preterm birth, and stillbirth. To fill this knowledge gap, we sought to determine the relationship between elevated outdoor temperature and the odds of placental abruption in a large population of pregnant women exposed to heat during warm months.

Section snippets

Study population

We included hospital records for all women whose pregnancies were complicated by placental abruption in Quebec, Canada from 1989 to 2012, and counted multiple pregnancies only once. We obtained the data from the Maintenance and Use of Data for the Study of Hospital Clientele registry, which contains hospital discharge information for all pregnant women who delivered in a hospital in the province (Ministry of Health and Social Services, 2017). In Quebec 99% of deliveries occur in hospital, thus

Results

There were 17,172 cases of placental abruption, including 5294 preterm and 11,878 term abruptions (Table 1). 19.2% of all abruptions occurred when the maximum weekly temperature was 30 °C or higher, while 13.7% of all weeks between May and October had temperatures that high. Slightly more term abruptions occurred at temperatures 30 °C or higher compared with preterm abruptions (19.4% vs. 18.8%). Most placental abruptions occurred among women between 25 and 34 years (63.6%), and women who had no

Discussion

In this case-crossover study of > 17,000 cases of placental abruption, heat was associated with a greater odds of placental abruption, particularly for women who were at term. Associations were stronger for pregnant women younger than 35 years of age, and women who were nulliparous or socioeconomically disadvantaged. There was no association between heat and placental abruption before term. These findings suggest that heat may trigger placental abruption in women who are at the end of gestation

Conclusions

In conclusion, this case-crossover study found that elevated temperature was associated with an increased odds of placental abruption, particularly abruption at term. Relationships were stronger for women younger than 35 years, and women who were nulliparous or socioeconomically disadvantaged. Placental abruption is an obstetric emergency with potentially serious consequences. Triggers are unclear especially for women without risk factors. The possibility that heat may increase a woman's

Funding

This work was supported by the Canadian Institutes of Health Research (MOP-130452). NA was supported by the Fonds de recherche du Québec-Santé (34695). The funders were not involved in the design and conduct of the study; collection, management, analysis, and interpretation of the data; and preparation, review, or approval of the manuscript.

Conflicts of interest

The authors report no conflict of interest.

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