Humans need iodine to make thyroid hormones which control the body's metabolism and are essential for normal growth and development. The relationship between iodine intake during pregnancy and birth outcome has not been determined. To assess this relationship, a prospective cohort study of 361 women was undertaken at Chiang Mai Province, Thailand. Casual urine samples collected several times throughout pregnancy, and urinary iodine excretion (μg/g creatinine) was used to assess the dietary iodine intake during pregnancy. Casual urine samples were collected from these pregnant subjects at some time from 9 to 12 A.M. and analyzed for creatinine and iodine. Methods for assessing urinary iodine and creatinine excretions were developed. The methods were reproducible and accurate. Birth outcome [birthweight, length, head circumference (HC), chest circumference (CC) and the percentage of head circumference to length (RHL)] and other factors which may affect birth outcome were recorded. These included social economic status, nutrient intakes and food frequency, maternal age, maternal anthropometry and blood pressure, and gestational age. The results from this study showed that dietary iodine intake (during 11-15 weeks of gestational age) was related to the infant's RHL. There is a clear mechanistic basis to explain these epidemiological findings.