Infants of insulin dependent (class B and above) diabetic mothers (IDM's) have a high rate of neonatal hypocalcemia (NHC) and hypomagnesemia. We carried out this study to test the hypotheses that: (1) infants of gestational diabetic (class A) mothers (IGDM's) are also at risk for NHC and (2) NHC in IGDM's relates to decreased whole blood Mg(2+) concentration. Thirty one term infants born to gestational diabetic mothers of classes A1 (diet controlled, n = 23) and A2 (requiring insulin, n = 8) of White's classification, were compared at 24 +/- 2 hours of age to 32 healthy, appropriate for gestational age controls, born after uncomplicated pregnancy, labor and delivery. Whole blood Mg(2+) and Ca(2+) were measured using an ion-specific electrode (Nova 8, Nova biomedical, Waltham, MA). The rate of NHC was higher in the IGDM group as compared to the control group (9 out of 31 [29%] vs. 1 out of 32 [3.1%] infants; p < 0.01). Whole blood Mg(2+) was lower in the IGDM group than in controls (p < 0.05). In multiple regression analysis, when Ca(2+) was used as the dependent variable and Mg(2+), diabetes class, gestational age, macrosomia and one minute Apgar scores were the independent variables, only Mg(2+) and diabetes class were significant (R2 = 0.4374; p < 0.01). Our results are consistent with the theory that Mg deficiency plays a role in NHC encountered in IGDM's, similar to what occurs in infants of insulin-dependent diabetic mothers.