The impact of maternal factors on the association between temperature and preterm delivery

Abstract

Background Few studies have examined maternal modifiers of temperature and adverse birth outcomes because of lack of data. We assessed the relationship between apparent temperature, preterm delivery (PTD) and maternal demographics, medical conditions, and behaviors. Methods A time-stratified case-crossover analysis was conducted using for 14,466 women who had a PTD (20 to less than 37 gestational weeks) from 1995 to 2009 using medical records from a large health maintenance organization in Northern California. Effect modifiers considered by stratification included several maternal factors: age, race/ethnicity, depression, hypertension, diabetes, smoking, alcohol use, pre-pregnancy body mass index, and Medicaid status. Difference by infant sex were also considered. Apparent temperature data for women who had a monitor located within 20 kilometers of their residential zip codes were included. All analyses were stratified by warm (May 1 through October 31) and cold (November 1 through April 30) seasons. Results Every 10°F (5.6°C) increase in average cumulatively weekly apparent temperature (lag06), a greater risk was observed for births occurring during the warm season (11.63%; 95% CI: 4.08, 19.72%) compared to the cold season (6.18%; −2.96, 16.18%), especially for mothers who were younger, Black, Hispanic, underweight, had Medicaid prior to giving birth, or had pre-existing or gestational hypertension or diabetes. Conclusions Our findings suggest that warmer apparent temperatures may exacerbate the risk of PTD, particularly for subgroups of women at higher risk.

Temperature and Preeclampsia: Is the Association Valid?

“…Thus, future research should explore multiple lag times and temperature exposure windows, while also considering metrics for mean, minimum, and maximum temperature. The effect of temperature and humidity is also important rather than temperature alone, since their combined exposure has been shown to increase preeclampsia risk . With prior evidence supporting the risk of preeclampsia from air pollutants, they qualify as potential confounders.…”

“…However, stratification by early and late preeclampsia would be the ideal approach, assuming there are a sufficient number of cases in each stratum, and control groups with comparable exposure windows are available. In a recent study of temperature and preterm delivery, 8 the study population was limited to cases between 20 and 36 gestational weeks, so gestational length was less of an issue when examining preeclampsia specifically, although some variation in risk by gestational week could still exist.…”

“…The effect of temperature and humidity is also important rather than temperature alone, since their combined exposure has been shown to increase preeclampsia risk. 8 With prior evidence supporting the risk of preeclampsia from air pollutants, 9 they qualify as potential confounders. Perhaps there is strong biologic plausibility that these covariates may actually be interesting effect measure modifiers of the association between temperature and preeclampsia.…”

Time series analysis of total and direct associations between high temperatures and preterm births in Detroit, Michigan

“…Case-crossover analysis is commonly used in studies of PTB and temperature 12 13 34. Because PTB is not a rare event in this particular population, the case-crossover ORs would not approximate risk ratios.…”

“…Studies in cities in Australia, Quebec, China, Belgium, Italy, Spain and across the USA have all found significant positive associations between hot temperatures within the week preceding delivery and PTB. 5 6 8–16 High ambient temperatures in the month of conception and the third trimester were also positively associated with PTB in Changsha, China 17 18. However, earlier, rigorously conducted time series analyses of temperature and PTB in Germany and in London, England, did not find associations 19 20.…”