Independent and joint effects of ambient temperature and relative humidity on fetal distress: effect modification by air pollutants.

Environmental factors are important factors affecting the maternal and fetal health, but there is a lack of evidence on the association between meteorological factors and fetal distress (FD). Aim to estimate the association between mean temperature (MT) and relative humidity (RH) and the risk of FD and evaluate the effect modification by air pollutants, we conducted a time-stratified case-crossover design, including 313 pregnant women with FD in Xi'an, China. Exposure-response relationship between meteorological factors and FD was estimated by using the distributed lag nonlinear model (DLNM). We further evaluated the association between meteorological factors and FD in the third trimester using a case-control design. Results showed that there was no significant association between MT and the risk of FD. RH was significantly positively associated with the risk of FD. For each 1% increase in RH, the odds ratio (OR) of FD at lag 4 and lag 014 were 1.017 (95% confidence interval (CI): 1.008, 1.027) and 1.042 (95%CI: 1.023, 1.061), respectively. In the third month of the third trimester, RH was significantly associated with FD (OR: 1.018, 95%CI: 1.003, 1.034). Stratified analyses showed that the effect estimates of MT were not significant in almost all subgroups, while the association between RH and the risk of FD was stronger among women admitted in the cool season during 0-14 days and women over 30 years old in the third month of the third trimester. The additive and multiplicative interaction effects of meteorological factors and air pollutants on FD were not statistically significant. In summary, our study provides the first evidence for the association between meteorological factors and FD, which provides a new perspective for reducing the occurrence of FD.