["American Journal of Human Biology, Volume 38, Issue 5, May 2026. ", "\nAmbient temperature during pregnancy is associated with birth length.\n\nABSTRACT\n\nObjectives\nThe impact of ambient temperature on fetal growth was tested using a data set of 1 208 491 singleton births taking place in Austria between 1984 and 2014. It was hypothesized that higher ambient temperature during pregnancy would be associated with shorter birth length.\n\n\nMethods\nDaily ambient temperature data spanning 1983 to 2014 from 39 Austrian districts were retrieved and used for analysis. A Dynamic‐Lag‐Non‐linear (DLNM) model was then fitted to link birth length with the time course of ambient temperature during each pregnancy. This model was adjusted for the following variables: maternal education level, maternal age, newborn sex, gestational age, newborn year of birth, maternal parity, and citizenship.\n\n\nResults\nThe effect of temperature on birth length was observed to be negative immediately following conception up to approximately gestational week 9 and again from about gestational week 15 toward the end of pregnancy. In contrast, during gestational weeks 10 to 14, there was no apparent association between temperature and birth length. The effect of hypothetically shifting a randomly drawn newborn's original birth date forward from January to October is associated with a decrease in predicted birth length by about 0.8 mm.\n\n\nConclusion\nHigher temperatures during early pregnancy and from the 15th gestational week onwards appear to have a negative effect on the fetal growth process.\n\n"]