BACKGROUND: Early-onset preeclampsia, traditionally defined as presenting before 34 gestational weeks, is associated with even higher risks of perinatal death, placental abruption, and stroke, than late-onset preeclampsia. OBJECTIVE: We estimated the degree of misclassification in a high-risk population of lupus pregnancies and a general population comparator when gestational age at delivery defined preeclampsia phenotype compared to first preeclampsia diagnosis. METHODS: Patients with lupus and general population comparators from Sweden with ≥1 singleton pregnancy in the Medical Birth Register with a documented ICD code for preeclampsia were included (2002-2016). We used gestational age at delivery (<34 versus ≥34 weeks) to phenotype preeclampsia early- versus late-onset and then reclassified based on first preeclampsia diagnosis date in the Patient Register. We cross-tabulated the two definitions and calculated sensitivity using the visit-based definition as the reference standard for general population and lupus pregnancies, overall and among nulliparous women. RESULTS: 331 pregnancies were diagnosed with preeclampsia, of which 322 were in both registers. Of those, 58 were early-onset based on gestational age at delivery (n = 29 in lupus pregnancies). Overall, 9% of early-onset preeclampsia in lupus (sensitivity 91%, 95% confidence interval [CI] 75, 98) was misclassified as late-onset compared to 19% in the general population (sensitivity 81%, 95% CI 64, 92). We noted similar misclassification (4% vs 22%) among nulliparous women. CONCLUSIONS: In the general population, early-onset preeclampsia was more likely misclassified as late-onset than in the high-risk lupus population. Relying on gestational age at delivery to phenotype preeclampsia, this way underestimates the occurrence of early-onset preeclampsia. This also suggests that the burden of early-onset preeclampsia as a public health concern may be under-reported, although this may be more applicable to milder preeclampsia where expectant management is employed. Research of biological and maternal predictors of early-onset preeclampsia may be dealing with differentially misclassified outcomes or samples.