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.