Could a uterus polym of 0.5mm cause a problem during pregnancy? A miscarriage? Should it be removed or its too small it might go away with pregnancy

Yes. It will not go away on its won. It can lead to abnormal bleeding, or infertility or an early miscarriage. But it cannot be removed once you are pregnant. The size is not the issue a small polyp can cause just as many problems as a large one from inflammation of the uterus.