What is the cause of iufd at 29 weeks, it was sudden it happens within 2 days?

Many things. If this is you I am very sorry for your loss. There can be many reasons for a spontaneous fetal demise this late in pregnancy. It could be due to a cord accident - tight cord around the neck or a knot in the cord. It could be due to an infection or a blood clot in the cord. The placenta could separate prematurely - known as an abruption. Sometimes the baby has a severe genetic issue.