Objective: Pregnancy is a physiological condition that its concurrence with fasting introduces some controversies about condition of mother and fetus. This study was conducted to evaluate the effect of fasting on pregnancy outcome.

Methods: The historical cohort paradigm of this study was conducted on referrals of one of the Tehran's hospitals in 2004. All pregnant women at one of the trimesters in holy month of Ramadan were included in the study. The women were divided into non-fasting, 1-10 days fasting, 11-20 days fasting, and 21-30 days fasting. For statistical analysis of data, covariance analysis and SPSS package was used.

Findings: In this study, 189 cases were evaluated and their mean age, weight, and body mass index (BMI) were 25.9 years, 61.7 kg, and 23.9 kg/m² respectively. The mean for number of days on fasting was 13 days and 66 cases (34.9%) had not been on fasting. In addition, there was no significant difference between BMI at the beginning of pregnancy, mother's age, number of pregnancies, and a history of abortion in different groups. Meanwhile, there was also no significant difference between means of weight, height, and head circumference of infants with number of days on fasting. Furthermore, there was no significant difference between pregnancy outcome parameters and fasting at different trimesters.

Conclusion: According to these findings, in healthy women with appropriate nutrition, Islamic fasting has no inappropriate effect on intrauterine growth and birth-time indices. Meanwhile, relative risk of low weight birth was 1.5 times in mothers on fasting at first trimester as compared to non-fasting mothers.