Vitamin D deficiency in pregnancy has been associated with decreased fetal growth, but previous studies have found no direct relation between the weight of the new-born child and the maternal serum level of 25-hydroxyvitamin D3 (calcidiol). The aim of this study was to evaluate the relation between maternal serum calcium and parathyroid hormone with reduced fetal growth in vitamin D deficient pregnant women. Thirty Pakistani women were included in the study at delivery. Only mothers without known chronic diseases who delivered vaginally after an uncomplicated pregnancy were included. Anthropometric data were recorded, and blood samples were drawn from the mothers 1-4 h after delivery. Nearly all (29/30) the Pakistani women had low (< 30 nmol/l) serum levels of 25-hydroxyvitamin D3. Thirteen of the mothers had high serum parathyroid hormone (PTH) levels (> 5.5 pmol/l). The median (range) level of ionised calcium in serum was 1.23 (1.15-1.28) nmol/l. A positive correlation was found between the level of ionised calcium in maternal serum and the crown-heel length of the infant (Spearman's rho = 0.65, P = 0.002, n = 20). The maternal serum PTH was related inversely to the crown-heel length (Spearman's rho = -0.47, P = 0.01, n = 30). No confounding effect of gestational age, sex of the infant, maternal height and body mass index (BMI) was found. The study indicates that vitamin D deficiency affects fetal growth through an effect on maternal calcium homeostasis.