A reduction in preterm births has been observed in Haguenau (Eastern France) during a 12-year intervention study with a program for prevention of preterm deliveries. The Perinatal Study of Haguenau was an observation tool used in a stable population, and it allowed measurement of the way women have progressively responded to the new proposals in prenatal care. It also allowed measurement of the results of the interventions: low birth weight (less than 2,500 g) and preterm birth rates (less than 37 weeks of gestation) among single live births. The total duration of the study was divided into three periods of four years (1971 through 1974, 1975 through 1978, and 1979 through 1982), for which the numbers of single live births are 5,763, 4,957, and 5,919, respectively. For the same periods, the low-birth-weight rates, 4.6%, 4.0%, and 3.8%, respectively, showed a significant decrease (P less than .001). Following a similar pattern, the rates of preterm birth were 5.4%, 4.1%, and 3.7% (a significant reduction with P less than .001). These improvements in pregnancy outcome do not disappear after standardization of mother's age, high blood pressure, or social class distribution. These findings, which concur with the results of others, enhance the hypothesis of a direct relationship between a prevention program and a reduction in preterm birth rates.