Why did my fetus die in less then a week I was 14 weeks pregnant? I just went to the doctor last week I had a sonogram and the baby had an amazing heart beat and this week the fetus is dead.

I . I am so sorry that this happened to you. We usually never know why a miscarriage occurs. There have been large studies done on this subject, and usually there is nothing wrong with the mom or the baby, but for some reason the pregnancy just doesn't "take". It may be an issue with the hormones or the placenta. Rest assured, there is nothing you can do differently at this stage of the pregnancy, but often women blame something that they have done which isn't true. Discuss this with your doctor also, and if you have had more than one miscarriage you may need a workup, but many women have one miscarriage in their lives and have no difficulty with subsequent pregnancies. Again, I am so sorry that this happened to you.
Hard to say. Sorry for your loss. This does happen far too often even when people do everything right. If the option comes up you can have them analyze the remaining tissue & structures to see if anything specific is present. At 14 wks there is a bit more room for them to flip around & get tied up in their cord. I've cared for liveborns with 3 true knots in the cord. Most times we don't know why.