Many Muslim women worldwide are pregnant during Ramadan and adhere to Ramadan fasting during pregnancy. In the present study, we determined whether maternal adherence to Ramadan fasting during pregnancy has an impact on the birth weight of the newborn, and whether the effects differed according to trimester in which Ramadan fasting took place. A prospective cohort study was conducted in 130 pregnant Muslim women who attended antenatal care in Amsterdam and Zaanstad, The Netherlands. Data on adherence to Ramadan fasting during pregnancy and demographics were self-reported by pregnant women, and the outcome of the newborn was retrieved from medical records after delivery. The results showed that half of all the women adhered to Ramadan fasting. With strict adherence to Ramadan fasting in pregnancy, the birth weight of newborns tended to be lower than that of newborns of non-fasting mothers, although this was not statistically significant ( − 198 g, 95 % CI − 447, 51, P= 0·12). Children of mothers who fasted in the first trimester of pregnancy were lighter at birth than those whose mothers had not fasted ( − 272 g, 95 % CI − 547, 3, P= 0·05). There were no differences in birth weight between children whose mothers had or had not fasted if Ramadan fasting had taken place later in pregnancy. Ramadan fasting during early pregnancy may lead to lower birth weight of newborns. These findings call for further confirmation in larger studies that should also investigate potential implications for perinatal and long-term morbidity and mortality.