Objective In Marfan syndrome, pregnancy is considered as high-risk due to connective tissue insufficiency and increased risk of aortic dissection. The objective was to demonstrate the consequences on maternal health, including aortic events in women with Marfan syndrome. Furthermore, to investigate outcome in undiagnosed with Marfan syndrome at the time of pregnancy. Design Information on aortic operations, aortic dissections, and pregnancy related outcome, from a national cohort of women with Marfan syndrome (n=195) were compared to an age-matched background population (n=19,500). The women with Marfan syndrome were divided according to whether the Marfan syndrome diagnosis was known at the time of pregnancy or not. Setting National health care registers Methods Main outcomes measures Registered events from the National Patient Registry in Denmark Results Significantly fewer women with Marfan syndrome gave birth, compared to the background population. In Marfan syndrome, complications related to cervix were increased (HR: 19.8 (95% CI: 2.2-177.5)) and the number of caesarean sections was higher (HR: 2.09 (95% CI: 1.41-3.08)). No women with known Marfan syndrome had a pregnancy related aortic dissection and the consequences of pregnancy and delivery on future aortic events seemed limited. Among undiagnosed women with Marfan syndrome there were more foetal deaths and all delivery-related dissections came from this subgroup. Conclusion The surveillance program for pregnant women with Marfan syndrome seems appropriate and only women with undiagnosed Marfan syndrome experienced a pregnancy-related aortic dissection. However, there is still an increased risk of complications during pregnancy, but the overall outcome data are quite reassuring.