- It is unclear whether idiopathic oligohydramniosis is associated with hot weather. One small study in a large European city reported a substantially higher incidence of oligohydramnios during a heat wave. The higher incidence appeared to be related to dehydration. The frequency of oligohydramniosis may also vary among different ethnic groups. The Bedouin population has much greater exposure to the hot and arid climate of the desert in small remote settlements. In contrast, the Jewish population—representative of any Western society—tends to live in urban areas with benefits such as air-conditioning and other comforts. The major aim of this retrospective population-based study was to determine the risk of oligohydramnios during the summer season in comparison to the rest of the year among a population of Jewish and Bedouin mothers who delivered at a tertiary university medical center in southern Israel. A secondary aim was to compare the frequency of oligohydramnios in different seasons among these 2 ethnic groups. The study was conducted between 1988 and 2007. Jewish women comprised 55.7% and Bedouin women 44.7% of the study population. The basic definition of oligohydramnios was an amniotic fluid index of 5 cm or less. A total of 191,558 deliveries occurred during the study period. Of these, 4335 (2.26%) were diagnosed with oligohydramnios: 1553 deliveries in the summer months and 2782 during the rest of the year. The rate of oligohydramnios in the summer months was higher than in the rest of the year (2.5 vs. 2.1%); the odds ratio (OR) was 1.18, with a 95% confidence interval (CI) of 1.1 to 1.3, P < 0.001. Multiple logistic regression analysis showed that the summer season was an independent risk factor for oligohydramnios (OR, 1.1; 95% CI, 1.02–1.21, P < 0.001). Bedouin ethnicity was another independent risk factor for oligohydramnios (OR, 1.3; 95% CI, 1.2–1.4, P = 0.015). There was also a significant trend among the Jewish population for an increased incidence of oligohydramnios during the summer months in comparison to all other seasons, but the difference was less marked than with the Bedouins. These findings show that oligohydramnios is significantly more common during the summer in comparison to other seasons of the year. Both the summer months and Bedouin ethnicity are independent risk factors for oligohydramnios.