Objectives  Hereditary haemorrhagic telangiectasia (HHT) affects 1 in 5–8000 individuals. Pregnancy outcomes are rarely reported. The major reason is that most women do not have their HHT diagnosed prior to pregnancy. Using a large well‐characterised series, we studied all pregnancies known to have occurred in HHT‐affected women, whether or not their diagnosis was known at the time of pregnancy. Our aim was to estimate rates and types of major complications of HHT in pregnancy, to guide management decisions. Design  Cohort study, with prospective, retrospective and familial components. Setting/Population  Tertiary referral centre population. Methods  All 262 pregnancies in the 111 women with HHT and pulmonary arteriovenous malformations (PAVMs) reviewed between 1999 and 2005 were studied. Eighty‐two women (74%) did not have a diagnosis of HHT/PAVM at the time of pregnancy. 222 pregnancies in their 86 HHT‐affected relatives were also studied. Main outcome measures  PAVM bleed, stroke and maternal death. Results  Thirteen women experienced life‐threatening events during pregnancy: 1.0% (95% CI 0.1–1.9) of pregnancies resulted in a major PAVM bleed; 1.2% (0.3–2.2%) in stroke (not all were HHT related); and 1.0% (0.13–1.9%) in maternal death. All deaths occurred in women previously considered well. In women experiencing a life‐threatening event, prior awareness of HHT or PAVM diagnosis was associated with improved survival (P = 0.041, Fisher’s exact test). Conclusions  Most HHT pregnancies proceed normally. Rare major complications, and improved survival outcome following prior recognition, means that pregnancy in a woman with HHT should be considered high risk. Recommendations for pregnancy management are provided.