Did you watch the movie The Princess Diaries? Anne Hathaway has correctly shown how 'princesses' and ladies are supposed to sit and wave. I did practice it a lot! However, it's so difficult to break this bad habit of crossing my legs at the knees, especially when you sit at the desk for 9 hours a day.
Many people, both men, and women do this unconsciously and often think of it as a 'ladylike' pose with no adverse consequences. However, research shows that crossing legs can increase your blood pressure and lead to a bad posture.
When you put one knee over the other, all the blood goes up to the chest instead, resulting in an increased quantity of blood being pumped out of the heart, raising blood pressure.
Sitting for long periods with your leg over your knee can cause your pelvis to rotate and tilt. A study showed that sitting with legs crossed for over three hours a day could result in a slouched posture, pain in the lower back, and neck pains. Also, prolonged leg-crossing eventually makes the inner thigh muscles shorter and the outer thigh muscles longer, which leads to a misalignment of your spine over time. When you have improper posture, your muscles are forced to compensate. This means they work harder than they need to, which can lead to pain and stiffness.
Of course, sitting with crossed legs won't cause a medical emergency. However, it can cause a temporary increase in your blood pressure and lead to poor posture. Try not to sit for longer than 1 hour without breaks and pay attention to the position of your legs. Your body is created to move, not to sit!