Reflecting a growing concern with high rates of urban growth in less developed countries, increasing attention focuses on the fertility of migrants to such places. The present study explores the relations between fertility, migration, and urbanization, using ‘own children’ data from the 1970 Census of Thailand. Information on ‘own children’ less than one year old is used to approximate fertility levels in the year before the census, and that on ‘own children’ aged 1–4 fertility levels during the four preceding years. Since migration was defined as a move to current residence between 1965 and 1970, the statistics on ‘own children’ less than five years old allow scrutiny of fertility during the period immediately before and after migration. Controlling for a variety of socio-economic characteristics of currently married women aged 20–49, and using several analytical approaches, the current fertility of migrants was consistently found to be higher than their own earlier fertility and higher than that of non-migrants in urban areas. Conversely, past fertility of migrants was lower than that of non-migrants, and past fertility levels of non-migrants were generally as high as current levels. The statistics thus suggest that migration is either selective of women with low fertility and/or that the migration process itself disrupts childbearing. Once the migrant women have moved, a ‘catch-up’ effect seems to operate to raise current fertility levels, but the cumulative fertility of migrants remains below that of non-migrants.