Difference in birth weight z-score associated with county-specific temperatures above the 90th percentile and county-specific temperatures below the 10th percentile during the entire pregnancy among 29,597,735 term births in 403 U.S. counties between 1989 and 2002, overall and by geographic region and climate zone. Models were adjusted for maternal age, race, marital status, years of education, smoking status, alcohol drinking, parity, chronic hypertension, and year and month of conception. Odds ratios were relative to temperatures ranging from 40th to 50th percentile of each county. CI, confidence interval.