Using a novel ion-selective electrode for ionized Mg (IMg2+), we sought to: (1) determine levels and fractions of IMg2+ in umbilical vessels of normal pregnant women: (2) determine their relationships with ionized calcium (ICa2+) levels: (3) determine whether any demographic or clinical parameters affect these levels of divalent cations: and (4) compare levels of IMg2+, total Mg, percent IMg2+, and ICa2+ with sera of normal pregnant and nonpregnant women. We obtained umbilical arterial and venous serum from 38 normal subjects and venous samples from 26 normal pregnant women and 42 nonpregnant women. The mean umbilical venous IMg2+ level (0.51 ± 0.01 mmol/liter) was significantly greater than the umbilical arterial level (0.48 ± 0.01 mmol/liter, p 2+ fractions were similar in umbilical arteries and maternal venous blood. IMg2+ levels were lower in multiparous patients and those who received epidural analgesia and higher in patients who had operative vaginal delivery and in Asians. Ionized fractions of Mg in umbilical veins were higher in large for gestational age infants. Our findings provide evidence of a Mg deficiency in pregnancy and suggest that measurement of IMg2+ may have clinical value for diagnosis and therapy of such deficiency. Furthermore, we have shown, for the first time, thelMg2+ levels differ between the umbilical artery and vein, so that caution must be used in interpretation of mixed cord blood samples. Finally, the present study provides a basis of comparison for future investigations of IMg2+ levels in complicated pregnancies.