BACKGROUND: We assessed the effect of mean ambient outdoor temperature during gestation on birthweight. OBJECTIVE: To assess the effect of mean ambient outdoor temperature during gestation on birth weight. DESIGN: Birth cohort study with record linkage to climate databases. SETTING: Aberdeen, Scotland. SAMPLE: A total of 12,150 individuals born in Aberdeen, Scotland between 1950 and 1956. METHODOLOGY/PRINCIPAL FINDINGS: Perinatal data from a cohort of 12,150 individuals born in Aberdeen, Scotland between 1950 and 1956 were linked to daily outdoor temperature data. Birthweight was seasonally patterned, with lowest birthweights among those born in the winter months (December-February) and highest birthweights among those born in the autumn months (September-November); P= 0.01 for joint sine-cosine functions. Mean ambient outdoor temperature during the first trimester of pregnancy was inversely associated with birthweight and mean ambient outdoor temperature during the third trimester of pregnancy was positively associated with birthweight. In fully adjusted (for sex, maternal age, birth year, birth order and social class) models a 1 degrees C increase in mean ambient outdoor temperature in the mid 10-day period of the first trimester was associated with a 5.4-g (95% confidence interval [CI] 2.9, 7.9 g) decrease in birthweight, whereas a 1 degrees C increase in the mid 10-day period of the third trimester was associated with a 1.3-g (95% CI 0.50, 2.1 g) increase in birthweight. Ambient outdoor temperature in the first trimester of pregnancy explained the seasonal patterning of birthweight. MAIN OUTCOME MEASURE: Birthweight. RESULTS: Birthweight was seasonally patterned, with lowest birthweights among those born in the winter months (December-February) and highest birthweights among those born in the autumn months (September-November); P= 0.01 for joint sine-cosine functions. Mean ambient outdoor temperature during the first trimester of pregnancy was inversely associated with birthweight and mean ambient outdoor temperature during the third trimester of pregnancy was positively associated with birthweight. In fully adjusted (for sex, maternal age, birth year, birth order and social class) models a 1 degrees C increase in mean ambient outdoor temperature in the mid 10-day period of the first trimester was associated with a 5.4 g (95% confidence interval 2.9, 7.9 g) decrease in birthweight, whereas a 1 degrees C increase in the mid 10-day period of the third trimester was associated with a 1.3 g (95% confidence interval 0.50, 2.1 g) increase in birthweight. Ambient outdoor temperature in the first trimester of pregnancy explained the seasonal patterning of birthweight. CONCLUSION: Higher ambient outdoor temperature in the first trimester of pregnancy and/or lower ambient outdoor temperature in the third trimester are associated with reduced offspring birthweight. With the increasing occurrence of temperature extremes, in particular, heat waves, these findings, if replicated in other studies, have important public health implications.