Background:Ramadan, the Islamic month of daytime fasting, is observed by many pregnant Muslims. Although pregnant women are exempt, many prefer to fast. Previous research has shown long-term adverse effects on various health outcomes among the offspring, but evidence on effects on perinatal outcomes is mixed. This study investigates effects of Ramadan during pregnancy among Muslims in the Netherlands.Methods:Data from the Perinatal Registry of the Netherlands (Perined) on all births between 2000 and 2010 to mothers recorded as Mediterranean (i.e. of Turkish/Moroccan descent, a proxy for Muslim) (n = 139,322) or as ethnically Dutch (n = 1,481,435) were used. Ramadan exposure was defined using an intention-to-treat approach as the occurrence of a Ramadan during gestation. Muslims with versus without a Ramadan occurring during gestation were compared using difference-in-differences analyses. In these multiple linear/logistic regressions, non-Muslims were additionally included in order to take out potentially remaining confounding through seasonal effects.Results:The occurrence of a Ramadan during pregnancy among Muslims was not associated with altered birth weight, gestational length, newborn's sex, perinatal mortality, low Apgar, or mild congenital anomalies. Odds for severe congenital anomalies were higher among the exposed (odds ratio: 1.17; 95% confidence interval: 1.00, 1.37), but this association became non-significant when adjusting for multiple testing.Conclusions:Despite earlier research showing long-term adverse health effects of prenatal exposure to Ramadan, there seems to be little or no relation between exposure to Ramadan during pregnancy and birth outcomes.