A Chinese research group investigated the effects of different temperatures on the number of preterm births and found that risk of preterm birth was higher in lower temperatures, and higher temperatures had a protective effect against preterm delivery.
Preterm birth (PTB), which is birth before the 37 gestational week, is a growing public health problem worldwide. It is the main cause of newborn and one of the leading causes of child death. However, the cause of PTB is unclear. It seems that PTB is a complex issue and genetic, behavioral, socio-economic and environmental factors play roles in its development. Environment factors, such as global warming, are frequently investigated in PTD research, but research findings are not consistent. Mostly, high temperature is thought to cause PTB by releasing hormones and causing dehydration that may lead to contractions and decrease in the uterus’ blood flow.
A Chinese study group recently published an article in the BioMed Central Environmental Health where they investigated the association between PTB and temperature changes in Shenzhen, China between 2005 and 2011. Birth-related data was obtained from the birth registry (1,040,638 births), and meteorological and air pollution data were also collected. Multiple pregnancies and cases where the gestational age was not certain were excluded. The average number of daily singleton PTBs was 22.85. The mean temperature was 23.14°C, ranging from 5.4 to 32.7°C. 5.61% of all births ended with preterm delivery, and the rate of PTB was lower in vaginal delivery, in boy newborns and in cases of women who were 20-34 years of age. They found that 9°C and 12.5°C were associated with a higher number of PTB, and the risk was higher at 9°C. Furthermore, a positive association was found with a 5, 25 and 30-day delay effect of 9°C temperature on PTB. However, higher temperatures had a negative association with PTB risk. It was also shown that younger mothers (younger than 19 years) were only sensitive to low temperatures, while in the case of mothers in the 20-34 and 35-49-year-old groups, higher temperatures seemed to be protective against PTB. The association between PTB and low temperatures was more prominent in boys and in cesarean sections.
Although previous studies have shown that higher temperatures may be an important factor in the development of preterm birth, this research group was able to prove the opposite: that lower temperatures were associated with more PTB cases and that higher temperatures even had a protective effect. The researchers noted that these findings are specific to the study population and that individual exposure to weather conditions might be miscalculated because of the widespread use of air-conditioners.
Written By: Dr. Fanni R. Eros