Women with insulin dependent diabetes mellitus are at increased risk for both first trimester spontaneous abortions and major congenital malformations when they become pregnant. The magnitudes of both of these risks depend upon the degree of metabolic control of their diabetes in the first trimester. The risks differ in the degree of control necessary to minimize them and the degree to which they can ultimately be reduced. A stricter degree of metabolic control is necessary to avoid spontaneous abortions than major malformations. Although the risks for both complications can be reduced by improved metabolic control, the risk for major malformations remains elevated, when compared to the risk for non-diabetic women, despite good to excellent control. In contrast, good to excellent control does reduce the risk for spontaneous abortions to a rate comparable to that seen in non-diabetic women. Women with insulin dependent diabetes mellitus who are planning pregnancies should be encouraged to achieve the best possible degree of metabolic control prior to and throughout pregnancy. They should be re-assured, however, that perfect control is not necessary to avoid dramatically increased risks for spontaneous abortions and major malformations.