- To assess seasonal patterns and monthly variations in the incidence of preeclampsia in the Jewish and Bedouin population in Israel, assuming that ethnicity and environmental factors may have a role in the causal mechanism. A retrospective population-based study comparing all singleton pregnancies of patients with and without preeclampsia was performed. The study included all deliveries between and including 1988 and 2007. For each month, the percentage of births complicated by preeclampsia was calculated. The relative risks of preeclampsia by month of delivery were estimated as odds ratios, using the month of August as the reference category. During the study period, 203,461 deliveries took place, of which 8,421 (4.1%) were complicated with preeclampsia (either mild or severe). Significantly higher incidence of preeclampsia was demonstrated in the Jewish population: 4.7% (n = 4,783 deliveries) versus 3.6% in the Bedouin population (n = 3,683), (p < 0.001). A similar U shaped trend in the incidence of preeclampsia was noted in both populations: the incidence was highest in the winter months and reached its nadir in August, with subsequent increase through the fall months (p < 0.001). Using August as the reference month, odds ratios were 1.32 (95% CI 1.19-1.48) for January, 1.38 (95% CI 1.24-1.54) for February and 1.33 (95% CI 1.19-1.48) for March. Time series analysis demonstrated a difference in the behavior of the 2 populations. Whereas the Jewish population had demonstrated a Gregorian-month cycle, the Bedouin population had demonstrated a lunar-month cycle. Preeclampsia is significantly more common in the Jewish population. However, both populations demonstrate the same seasonal pattern, with higher incidence of preeclampsia in the winter months. Our findings suggest that climate and environmental exposure may have an important role in the pathophysiology of preeclampsia.