The last menstrual period is used to estimate gestational age. This paper examines sources of measurement error related to the recall of last menstrual period among Mexican immigrant women living in the United States. Qualitative analyses (focus groups and cognitive interviews) suggest that last menstrual period recall does not seem to be a large source of measurement error in the calculation of gestational age and the impact of this type of error on the misclassification of preterm births appears to be minimal. Questions for querying about last menstrual period in this population are offered.