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Background Women with left ventricular outflow tract obstruction (LVOTO) are at risk of adverse pregnancy outcomes, which have been related to the severity of the LVOTO. However, the physiological adaptations of pregnancy increase the gradient across an LVOTO, often altering the clinical perception of risk. Here, we examined how the LVOTO gradient changes during pregnancy, investigating whether a greater increase in the gradient is associated with worse pregnancy outcome and whether any increase in the gradient is transient, reversing after the end of the pregnancy.Methods The study was conducted at three tertiary centres. Data were collected from 125 pregnancies in 100 women with a range of causes of LVOTO, including bicuspid aortic valve (n=71), trileaflet aortic stenosis (n=23), prosthetic valve replacement (n=18) and subvalvular stenosis (n=8). Measurements included echocardiographic parameters (peak and mean LVOT gradients, left ventricular ejection fraction) and pregnancy outcomes. The primary endpoints were the change in gradient and pregnancy outcomes.Results The mean (19.5±11.9 mm Hg to 23.8±14.9 mm Hg) and peak (31.6±19.2 mm Hg to 40.6±27.7 mm Hg) gradients increased significantly during pregnancy and in a minority of cases this rise persisted postpartum. Pregnancy complications were unrelated to baseline gradient severity or the increase in gradient observed during pregnancy. Counter to our hypothesis, there was no association between the increase in gradient and adverse pregnancy outcomes.Conclusions The physiological changes of pregnancy resulted in an increase in the gradient across the LVOTO, which was not associated with any adverse obstetric outcomes. In some cases, the increase in the gradient only partially reversed after pregnancy, implying that pregnancy may induce a structural change in the lesion, giving rise to LVOTO.