Ca status in the uterus during pregnancy has been suggested to affect fetal growth and size at birth. In Bangladesh, low Ca levels in pregnant women and low birth weight in infants are common. The present study explored the association between Ca levels in cord blood and newborn size at birth (birth weight and birth length) in Bangladesh. Samples and data included 223 women with live-born singleton deliveries in rural Bangladesh. Newborn weight and length were measured at birth. From cord blood obtained at delivery, Ca, 25-hydroxy vitamin D, bone-specific alkaline phosphatase and intact parathyroid hormone levels were determined. An association between size at birth and Ca levels in cord blood was found (birth weight, P = 0·022; birth length, P = 0·001). Associations between Ca and newborn size were further analysed using multivariate regression analyses. After adjusting for several covariates of characteristics in mothers and newborns (gestational weeks at birth, sex of newborn, socio-economic status, maternal height, BMI, age and season at birth), birth length still exhibited a significant relationship with Ca levels in cord blood (birth length, P = 0·030). The present study indicates that Ca status in cord blood might be associated with the birth length of newborns. Ca levels during gestation may affect fetal growth.