Can a person allowed to take a 20hrs flight while 36-38 week pregnant?

You should not: You should not travel far from your doctor at the end of pregnancy.

Not advised: I would not advised taking such a long flight that late in the pregnancy. You are an increased risk for blood clots while pregnant and that long of a period of inactivity could raise that risk as well. Also, you would not want to be that far from home that close to delivery.