OBJECTIVE To determine the prevalence of pregnancy complicated by diabetes in a representative sample of the U.S. population. RESEARCH DESIGN AND METHODS We analyzed data from a multistaged cross-sectional probability sample of live births recorded in the U.S. in 1988 for women 15–49 years of age. The main outcome measure was pregnancy complicated by diabetes. RESULTS Diabetes was present in congruent to 154,000 (4%) of all pregnancies in the U.S. Gestational diabetes mellitus (GDM) accounted for 135,000 of such pregnancies (88%), non-insulin-dependent diabetes mellitus (NIDDM) for 12,000 (8%), and insulin-dependent diabetes mellitus for 7,000 (4%). On average, the mothers with NIDDM (29.6 years) and GDM (29.3 years) were older than mothers whose pregnancies were not complicated by diabetes (26.2 years; P < 0.05). In multivariate analyses, the odds of having a pregnancy complicated by GDM increased significantly with maternal age and body mass index. CONCLUSIONS Pregnancy is complicated by diabetes more often than was previously believed. More frequent testing may further increase the apparent prevalence of GDM.