At one time, women that had heart disease (elevated blood pressure, heart defects from birth often noted by the presence of heart murmurs, irregular heartbeats, etc.) were advised not to get pregnant. This has changed because most women with heart disease can have successful, healthy pregnancies and healthy babies with some special guidance and planning.
Certainly, pregnant women with co-occurring heart/cardiac disease present special management issues but most of the issues can be overcome with proper medical care, diet, exercise, nutrition, and medication if prescribed by your doctor. Pregnancy is a stressful condition, sometimes emotionally for women and always it is always physically stressful. Your heart is called upon to provide nourishment for two so it needs to work significantly harder supplying blood to both you and your baby.