The present study aimed to determine the association between maximum apparent temperature during pregnancy with birth weight and preterm birth in newborns in the province of Piura, 2011–2016. For this purpose, regression models were generated considering 4 different exposure windows: entire pregnancy, and for each of the trimesters of gestation.
It was obtained that exposure to upper quartiles of maximum temperature during the entire pregnancy, and the third and second trimester, starting at 40°C, was associated with a decrease in birth weight, this effect being greater during the third trimester, where Q4 and P95 was associated with a reduction of 66.08 and 70.48 grams, meaning that different stages of pregnancy progression are more susceptible than others.
A recent meta-analysis (19) found that exposure to high temperature is associated with a decrease in birth weight in a range of (-39.4 to -15.0 grams) similar to our results, however, it is not specified whether ambient temperature or apparent temperature was used, as in our case. In a retrospective study performed in California, United States, where apparent temperature was evaluated similar to ours, it was found that the greatest effect of temperature on birth weight at term occurred in the third trimester when exposed to temperatures higher than 15°C (20).
It is postulated that one of the mechanisms by which temperature affects pregnancy is by promoting an inflammatory state of the placenta (21). It is possible that this mechanism is more relevant during the third trimester of gestation, since it is during this stage that the growth rate of the fetus is higher, thus explaining why we found a greater decrease in birth weight in this window of exposure, like other studies.
Regarding preterm birth, we found that there HImax affected overall gestation increasing in 93% the risk for preterm birth when exposed to temperatures > 45.7°C (P95); but there is a greater susceptibility during the first two trimesters of gestation since almost all groups of exposure showed increased hazard for preterm birth, but the greater effect of HImax was found in P95 in the third trimester. In a study conducted in Sabzevar, Iran, where maximum temperatures similar to ours were observed, the hazard of preterm birth was found to increase with temperatures above 30°C (22). On the other hand, the greater susceptibility found in the first 2 trimesters agrees with those obtained in a study performed in China where more than 1 million records were evaluated, finding that exposure to temperatures above 24°C was associated with a 78% increase in the hazard of preterm birth in the first trimester, and a 41% increase in the second trimester; while in the third trimester this effect was diluted (23). This increased susceptibility during the first trimesters of gestation seems to be a characteristic mainly given in areas with a higher basal temperature, since in an evaluation conducted in China (24), provinces considered as hot areas showed a greater increase in preterm birth hazard during this period, as well as during the first week of the third trimester of gestation.
It is possible that this increased susceptibility during the first trimesters of gestation is due to heat stress impacting placental formation, as suggested by a model in pigs (25). The inflammatory state would cause deficiencies in blood vessel formation, leading to inadequate oxygenation, and thus a constant oxidative environment incapable of sustaining a pregnancy to term (26). However, it has been observed that the risk of placental abruption associated with environmental heat stress occurs mainly in pregnant women at term (27); therefore, the mechanism could be different.
Although our study found significant associations between HImax birth weight and preterm birth, there are some limitations. The study design only allows us to assess exposure at the population level, creating a ecological bias. To avoid these biases, future studies should include individual measurements. Additionally, there are other factors that could modify exposure to temperature or heat stress, such as having refrigeration equipment at home or at the workplace; or if the pregnant woman migrated to other cities during different stages of pregnancy.
On the other hand, several studies have shown how pollutants, especially particulate matter (PM) < 10 and 2.5 µm and nitrogen oxides (NOx) significantly increase the risk of adverse birth outcomes (28–30); however, for the present study no information could be obtained regarding air pollutants. Likewise, social and urban factors may play an important role in terms of thermal sensation, as socioeconomic status and density of green areas interact with temperature effects (31).
On the other hand, it should be noted that most studies have been conducted in countries far from the equator, evaluating Asian, European, and American populations (12, 13). In these regions, environmental temperatures throughout the year would show considerable fluctuations. In addition, the monitoring station only reported daily minimum and maximum temperature, so it was not possible to evaluate the relationship with mean daily temperature. To our knowledge, our study is one of the first attempts to evaluate the effect of temperature on birth outcomes in an equatorial country, which helps to establish this area of research that is still novel in our region.
Since adverse birth outcomes can affect health in adulthood, increasing the risk of metabolic diseases, new methods for assessing the effects of temperature and climate variability in pregnancy, such as the novel approach used in the present study, are needed to contribute, first, to the understanding of biological effects and response to exposure, and second, to optimize prenatal care controls by considering other variables such as temperature.
Exposure to high maximum temperatures negatively affects fetal growth, especially during the third trimester of gestation; while it affects the duration of pregnancy mainly when exposure occurs during the first two trimesters. The thermal stress resulting from this exposure could be causing deficiencies at the placental level, both in the formation and functioning of the organ; however, these effects need to be explored in future studies given the current situation of climate change.
The findings of this study could help to understand how temperature may affect reproductive health in Piura, be useful to anticipate and adapt hospital obstetric services during periods of high heat; and promote public health initiatives and campaigns that could help improve the impact of temperature stress on birth outcomes. Finally, this study provides new insight into the assessment of temperature exposure during pregnancy, which is during the stage of greatest fetal growth, which could ultimately serve as a new tool for environmental epidemiology research and evaluation.