Among women of Mexican descent, increased acculturation in the US has been associated with poorer health behaviors during pregnancy. This study examined a population of low-income women of Mexican descent in an agricultural community to determine: whether social support patterns were associated with age at arrival in the US; whether social support was associated with pregnancy behaviors; and whether increased social support could prevent some of the negative pregnancy behaviors that accompany acculturation. Participants were 568 pregnant women enrolled in prenatal care in the Salinas Valley, California. Participants were predominantly Spanish speaking, born in Mexico, and from farmworker families. Information on social networks, social support, age at arrival in the US, and pregnancy health behaviors was gathered during interviews conducted during pregnancy and immediately after delivery. Poorer health behaviors were observed among women who had come to the US at a younger age. Social support during pregnancy was lowest among women who had come to the US at an older age. High parity, low education, and low income were also associated with low social support. Higher social support was associated with better quality of diet, increased likelihood of using prenatal vitamins, and decreased likelihood of smoking during pregnancy. High social support also appeared to prevent the negative impact of life in the US on diet quality. Women with intermediate or low levels of social support who had spent their childhoods in the US had significantly poorer diet quality than women who had spent their childhoods in Mexico. However, among women with high social support, there was no difference in diet quality according to country of childhood. Thus, in the case of diet quality, increased social support appears to prevent some of the negative pregnancy behaviors that accompany time in the US among women of Mexican descent.