Seasonal variation in pre-eclamptic rate and its association with the ambient temperature and humidity in early pregnancy.

Tam, Wing Hung; Sahota, Daljit S; Lau, Tze Kin; Li, Chi Yin; Fung, Tak Yuen

AIMS To determine any relationship between ambient temperatures adjusted for humidity at conception and the occurrence of pre-eclampsia. METHODS The subjects were singleton primiparae delivered in a Teaching Hospital between 1995 and 2002. We studied the odds of developing pre-eclampsia across months and investigated the association between the pre-eclamptic rates based on the months of conception and the mean monthly heat index. RESULTS A total of 245 (1.6%) women were diagnosed pre-eclampsia and eclampsia during the study period. There was a significant association between the seasons of conception and rate of pre-eclampsia (logistic regression Wald chi(2) = 9.2, p = 0.03). Conceptions during summer had a higher risk of pre-eclampsia than those during autumn (2.3 vs. 1.6%, OR 1.7, 95% CI 1.2-2.5). Women who conceived in June had the highest risk of developing pre-eclampsia (OR 2.8, 95% CI 1.5-5.2) while women who conceived in October had the lowest after adjusting for age. A 2-month time lag was observed between the peak pre-eclamptic rate in women who conceived in June and the peak heat index in August. CONCLUSION Singleton primiparous women who conceived in summer and had a longer exposure to higher ambient temperature were at a greater risk of pre-eclampsia.