There are relatively few low-weight births among Mexican Americans, despite their socioeconomic disadvantages. Fewer low-birth-weight (LBW) births result when babies are heavier at term or when there are fewer preterm deliveries. The authors used 1994 US singleton livebirth birth certificates to compare Mexican Americans with non-Hispanic Whites. They found that the lower LBW rate among Mexican Americans (5.8%) compared with non-Hispanic Whites (6.1%) occurred because fewer small, preterm babies were born to Mexican Americans (3.4% vs. 3.9%). This result was obscured by two findings. First, the mean birth weight of Mexican American babies (3,343 g) was lower than that of non-Hispanic White babies (3,393 g). This finding again showed the independence of mean birth weight and LBW. Second, the overall preterm birth rate was higher among Mexican Americans (10.6%) than non-Hispanic Whites (9.3%). Our hypothesis is that this finding reflects errors in recorded gestational age, as illustrated by a strongly bimodal birth-weight distribution at young gestational ages for Mexican Americans. Further studies on the LBW paradox among Mexican Americans should thus focus on gestational age more than on birth weight.