Methods and systems for fetal intervention to address malformations or other conditions are presented. In one instance, a method is provided that includes introducing a guide catheter into the fetus' venous system and using controlled magnets to position the guide catheter at a desired location. The method also involves positioning the treatment catheter at a treatment location using the guide catheter and delivering a treatment using the treatment catheter at the treatment location. In one particular application, a hypoplastic left heart is treated by using a balloon catheter as the treatment catheter and inflating a balloon on the balloon catheter at the aortic valve. Other methods and systems are disclosed.