To study the trends in gestational age and birthweight in France, we compared data from three surveys that were based on representative samples of births in 10 French regions and were conducted in 1972, 1981 and 1988-89. Only single livebirths were considered in the analysis. The rate of preterm births decreased from 7.9% in 1972 to 5.8% in 1981 and 4.0% in 1988-89. The corresponding rates of children with a birthweight below 2500 g were 5.4, 4.3 and 4.7%. The mean birthweight of preterm babies born in 1988-89 was lower than the mean birthweight of those born in 1972 and 1981. The reduction in preterm births cannot be attributed to the improved accuracy of gestational age determination, but the general use of ultrasound may have played a major role in the apparent reduction of the mean birthweight of preterm babies.