Objectives: To explore the impact of anti-hypertensive treatment of pregnancy-induced hypertension on foetal growth and hemodynamics in women with pre-existing diabetes. Methods: A prospective cohort study of 247 consecutive pregnant women with pre-existing diabetes (152 type 1 diabetes; 95 type 2 diabetes), where tight anti-hypertensive treatment was initiated and intensified (mainly with methyldopa) when office blood pressure (BP) ≥135/85 mmHg and home BP ≥130/80 mmHg. Foetal growth was assessed by ultrasound at 27, 33 and 36 weeks and foetal hemodynamics were assessed by ultrasound Doppler before and 1–2 weeks after initiation of anti-hypertensive treatment. Results: In 215 initially normotensive women, anti-hypertensive treatment for pregnancy-induced hypertensive disorders was initiated in 42 (20%), whilst 173 were left untreated. Chronic hypertension was present in 32 (13%). Anti-hypertensive treatment for pregnancy-induced hypertensive disorders was not associated with foetal growth deviation (linear mixed model, p = 0.681). At 27 weeks, mainly before initiation of anti-hypertensive treatment, the prevalence of small foetuses with an estimated foetal weight <10th percentile was 12% in women initiating anti-hypertensive treatment compared with 4% in untreated women (p = 0.054). These numbers were close to the prevalence of birth weight ≤10th percentile (small for gestational age (SGA)) (17% vs. 4%, p = 0.003). Pulsatility index in the umbilical and middle cerebral artery remained stable after the onset of anti-hypertensive treatment in a representative subgroup (n = 12, p = 0.941 and p = 0.799, respectively). Conclusion: There is no clear indication that antihypertensive treatment causes harm in this particular at-high-risk group of pregnant women with diabetes, such that a larger well-designed study to determine the value of tight antihypertensive control would be worthwhile.