There is a significant association between birth weight and cognitive test scores in childhood, even among individuals born at term and with normal birth weight. The association is not explained by the child's social background. Here we examine whether mother's cognitive ability accounts for the birth weight-cognitive ability association. We analysed mother and child data from the National Longitudinal Survey of Youth 1979. Random effects models were employed to utilise fully the repeated cognitive tests on the same child, and to include all children of each mother. Mother's score on the Armed Forces Qualification Test (AFQT) was significantly related to child's birth weight. Birth weight was significantly related to the child's scores on the Peabody Individual Achievement Test. This association was attenuated by up to two-thirds after taking into account mother's AFQT score. In this large sample the association between birth weight and cognitive ability was substantially explained by mother's IQ.