Aims/hypothesis: Hypertensive disorders are prevalent among pregnant women with pre-existing diabetes, but the prevalence and impact of white coat hypertension are unknown. Measurement of home BP before initiation of antihypertensive treatment is necessary to identify white coat hypertension since international guidelines recommend that white coat hypertension is left untreated. The aim of this study, conducted among women with pre-existing diabetes, was therefore to examine the prevalence of white coat hypertension in early pregnancy, and pregnancy outcome in women with white coat hypertension in early pregnancy. Methods: A prospective cohort study was undertaken involving women with pre-existing diabetes from a geographically well-defined area. Based on office BP in early pregnancy and home BP measured for 3 days, women were categorised in three groups: (1) white coat hypertension, defined as office BP ≥ 135/85 mmHg and mean home BP < 130/80 mmHg; (2) chronic hypertension, defined as pre-pregnancy hypertension including newly detected office BP ≥ 135/85 mmHg with home BP ≥ 130/80 mmHg; and (3) normotension. Office BP was measured every 2 weeks and, if ≥ 135/85 mmHg, home BP measurements were performed. White coat hypertension was left untreated, and tight antihypertensive treatment was initiated when both office BP ≥ 135/85 mmHg and home BP ≥ 130/80 mmHg. Pregnancy-induced hypertensive disorders were defined as office BP ≥ 140/90 mmHg with home BP ≥ 130/80 mmHg when available, with onset after 20 weeks of gestation. Results: In total, 32 out of 222 women with pre-existing diabetes had newly detected office BP ≥ 135/85 mmHg in early pregnancy. White coat hypertension was present in 84% (27/32) of these women, representing 12% (95% CI 8%, 17%) of the whole cohort. Chronic hypertension was present in 14% (n = 32) and normotension in 74% (n = 163). Women with white coat hypertension were characterised by higher pre-pregnancy BMI (p = 0.011), higher home BP (p < 0.001) and higher prevalence of type 2 diabetes (p = 0.009), but similar HbA_1c (p = 0.409) compared to women with normotension. Regarding pregnancy outcome, pregnancy-induced hypertensive disorders developed in 44% (12/27) of women with white coat hypertension in comparison with 22% (36/163) among initially normotensive women (p = 0.013), while the prevalence of preterm delivery was comparable (p = 0.143). The adjusted analysis, performed post hoc, suggested approximately double the risk of developing pregnancy-induced hypertensive disorders (OR 2.43 [CI 0.98, 6.05]) if white coat hypertension was present in early pregnancy, independently of pre-pregnancy BMI and parity. Conclusions/interpretation: White coat hypertension is prevalent in women with pre-existing diabetes and may indicate a high risk of later development of pregnancy-induced hypertensive disorders. To distinguish between persistent white coat hypertension and onset of pregnancy-induced hypertension, repeated home BP monitoring is recommended when elevated office BP is detected. The study was registered at ClinicalTrials.gov (ID: NCT02890836).