Marfan's syndrome is a dominant autosome transmitted disease with different degrees of seriousness. When it is associated with pregnancy there is a considerable increase in the rate of cardio-vascular complications which are responsible for a serious rate of maternal mortality due to aortic dissection. It is therefore very important that the patient, before starting a pregnancy, should be examined very carefully clinically and with ultrasound. A woman with dilatation of the aorta less then 40 mm as measured ultrasonically and who has no major clinical signs can start a pregnancy but even then she should be considered as a high risk case. In other cases therapeutic termination of pregnancy or prophylactic surgical treatment of the condition should be considered. Apart from the serious genetic risks (50%) the risks for the fetus are dominated by prematurity and by fetal death resulting from maternal death.