It's not true. No matter what you do, there's a 50-50 chance of getting either gender.
The reason people think it's true is if they try something and they happen to get whichever gender they hoped for they think it worked, then they tell their friends, and their friends try it, and they don't get the gender they hoped for, and then they think the reason is not that the process is fake, but rather that they did something wrong, in which case they try again the next time, and of course then they're back to a 50-50 chance of getting the gender they hope for, and no, I've never heard of a run-on sentence.