OBJECTIVE-To evaluate the incidence and predisposing factors of severe hypoglycemia (SH) in pregnant women with insulin-dependent (type I) diabetes mellitus.RESEARCH DESIGN AND METHODS-SH (impairment of consciousness due to hypoglycemia subsequently treated with glucagon or i.v. glucose) was recorded in all pregnant type I diabetic women (n = 77) who attended our pregnancy clinic during 85 pregnancies from 1986 to 1990.RESULTS-Ninety-four SHs were reported during 35 pregnancies. Of 94 SHs, 84% occurred before the 20th gestational wk (median 12th wk) and 77% during sleep. In the group with SH, there was no permanent maternal sequelae, and there was a favorable fetal outcome (no perinatal death and no congenital malformation). Mean HbA lc values were not different between the group with and without SH for the first half (6.4 ± 1.1 vs. 6.3 ± 0.9%) and 2nd half (5.4 ± 0.6 vs. 5.5 ± 0.7%) of pregnancy. The percentage of women with SH before pregnancy (51 vs. 28%, P < 0.05) and the incidence of SH patients before pregnancy (0.49 vs. 0.08 SH/ patient/yr) was different between the group with and without SH.CONCLUSIONS-SH is frequent during pregnancies of type 1 diabetic women with near normoglycemia. The risk for SH is particularly pronounced during the first half of pregnancy and in women with a history of SH.