How accurate is the Ramzi theory?

How accurate is the Ramzi theory?

I'd never heard of it until a week or so ago, but everyone seems to be able to come to the same conclusion more or less on a scan photo. How accurate is this theory? And how many weeks is it accurate up until? If it is accurate, why/how? It seems weird/cool to be able to tell gender based on the position of the foetus and stuff