Impact of the 25-Hour Religious Fast of "Yom Kippur" on Pregnancy Outcomes in Low-Risk Singleton Pregnancies in Israel: A Multicenter Retrospective Study.

Evidence regarding the impact of prolonged religious fasting during pregnancy is limited and inconsistent. We aimed to examine the effects of Yom Kippur fasting, a complete 25-h religious fast, on pregnancy outcomes, with a primary focus on preterm delivery, in low-risk singleton pregnancies in Jerusalem, Israel. We performed a multicenter retrospective cohort study of Jewish women aged 18-45 years with uncomplicated singleton pregnancies delivered in our maternity wards between 2006 and 2021. Data were extracted from electronic medical records. A preliminary survey estimated the prevalence of fasting during Yom Kippur at 92% (80% complete, 12% partial). Deliveries occurring on Yom Kippur and up to two days afterward were compared with deliveries on the corresponding days one week prior. The primary outcome was preterm delivery (< 37 weeks). Secondary outcomes included preterm birth <34, <32, and <28 weeks, other maternal and neonatal outcomes, and biochemical markers of maternal dehydration including urine ketones, blood urea nitrogen, hemoglobin, and hematocrit. During the study period, 5832 deliveries occurred across study and control days. After exclusions, 3943 deliveries (67.6%) were analyzed: 697 occurred on Yom Kippur, 1485 within one day and 2080 within two days following, and 1863 occurred on the three corresponding days the week before. Preterm delivery rates were comparable across groups (< 37 weeks: 2.2%, 1.9%, 2.2%, and 2.2% in groups 1, 2, 3, and control, respectively). Multivariate analysis revealed no significant differences in gestational age at delivery or preterm birth rates at various thresholds. Study group women exhibited higher hematocrit and urine ketone levels, without clinically significant adverse maternal and neonatal effects. Labor duration was shorter in fasting women, who also presented with greater cervical dilation, underwent fewer vaginal examinations, had lower epidural use, and reduced chorioamnionitis rates. However, the rate of shoulder dystocia was higher among fasting women. The 25-h Yom Kippur religious fast does not appear to increase the risk of preterm delivery or other adverse maternal and neonatal outcomes in low-risk singleton pregnancies. The observed association with shorter labor may reflect dehydration-related changes in maternal physiology. Further studies are warranted to assess the impact of prolonged fasting in high-risk pregnancies.