A growing body of evidence suggests that maternal diet during pregnancy can lead to permanent alterations to the physiology of the fetus. It is unknown whether intermittent maternal fasting during Ramadan has long-term associations with the offspring's body composition. By using data from the third wave of the Indonesian Family Life Survey (2000), we compared the body mass indices (weight (kg)/height (m)2) of Muslims who had been in utero during Ramadan with those of Muslims who had not been in utero during Ramadan. Adult Muslims who had been in utero during Ramadan were slightly thinner than Muslims who had not been in utero during Ramadan (adjusted adult body mass index: -0.32, 95% confidence interval: -0.57, -0.06). Those who were conceived during Ramadan also had smaller stature, being on average 0.80 cm shorter than those who were not exposed to Ramadan prenatally. Among non-Muslims, no such associations were found. This study suggests that exposure to Ramadan during pregnancy may have lasting consequences for adult body size of the offspring.