We argue that changes over time in how ideas of stress are incorporated into understandings of pregnancy and motherhood among Mexican immigrant women living in the United States may affect the documented increase of low birth weight infants born to those women. Stress has consistently been linked to low birth weight, and pregnant Mexican American and Mexican immigrant women differ in levels of perceived social stress. What is lacking is an explanation for these differences. We utilize a subset of 36 ethnographic interviews with pregnant immigrant women from northern Mexico and Mexican Americans living in south Texas to demonstrate how meanings of pregnancy and motherhood increasingly integrate notions of stress the longer immigrant Mexican women live in the United States. We situate our results within anthropological and sociological research on motherhood in the United States and Mexico, anthropological research in the U.S.–Mexico borderlands, and interdisciplinary research on Hispanic rates of low birth weight.