Investigation of magnesium (Mg) homeostasis has re-emerged as an area of interest in preterm born neonates who are at risk for brain pathology. However, data regarding the association between the biologically active ionized form of Mg and gestational age (GA) at an early stage of life in newborn infants are controversial.We evaluated the total and ionized Mg electrolyte (TMg and IMg) as well as the calcium (TCa and ICa) and pH in the cord blood and on day 2 of life in 22 neonates born at different gestational ages ( or =35 week) without magnesium tocolysis and absence of serious complications during pregnancy and delivery.The IMg fraction that accounted for 68.1+/-5.1% of the TMg in the cord blood and 67.9+/-4.5% of the TMg on day 2 of life, was significantly higher in very preterm infants (GA 35 week. Higher IMg levels were correlated with the lower pH that was recorded in the cord blood of the very preterm infants (correlation coefficient, r=-0.80, p< 0.0001) and ICa (r = -0.52, P< 0.01). Lower pH also was correlated with the GA (P< 0.0001). However, standard multiple regression analysis showed significant association between IMg levels and decreased pH but not the gestational age or ICa (beta=-1.10+/-0.21, p< 0.00009).Extremely preterm infants even without additional exposure to tocolytic magnesium are at risk for the lower pH associated elevation of ionized Mg, which should be considered during the management of these infants in order to prevent hypermagnesemia-related pathology.