Natural gender selection is nothing just to complicated, but it's pure science combined with randomness and other stuff like faith. It's happening all the time when there is a sperm cell that's going to fertilize the egg and the baby is going to get birth. The fact is that you may know by now that the sperm cells have two main characteristics:

• they do care 24 chromosomes - not just the x or the y, the first 23 are names autosomes and the last is the baby gender chromosome
• the baby gender chromosome is an y - for a boy, or an x - for a girl
• the egg from the mother also has 24 chromosomes, but besides the father it has only one baby gender solution - a girl, an x chromosome

   It works pretty simple - let's say that your father is going to ejaculate just in time and there are millions of sperm cells that are in there and rushing through the egg in order to catch it (the first that comes, is the first that is served). Think about it, there are millions of sperm cells, either with the x, either with the y, 50/50, so the chances of natural gender selection is that you either have a boy, either have a girl, 50/50. Nothing to complicated, nothing to stufy, just to understand the fact about it. 

   The fact is that science is once again there and it's helping people to get pregnant, to have babies, to choose their baby sex and whatsoever - the fact is that it's kinda unethical for you to choose your own baby sex, because in that way, the entire population would be controled - there are a few clinics in USA that provide this kind of material - I'll show you a movie, but the fact is that artificial gender selection is not going to have to work 100% and you're going to give a lot of money to. There are different ways to get your baby, a boy or a girl, as you'll see in the video below:

• as the movies says, this is just a theory, basicaly you have to know that it does not matter the time of intercourse (it may matter if you make it to much), but the fact is that it does not matter if the women is on top, gravity and some stuff. There are no scientific evidence, for this.