Choosing the sex of your baby is never a guaranteed thing. There are many theories and old wives tales on how to do this. They range from various sexual positions to eating certain types of foods, but realistically, none of them work you still have a bout a 50/50 chance.
If you are really serious about choosing the sex of your baby, then you should take a look at this site that has an almost 95% success rate.
Like I said, there are no guarantees, but every little bit helps.
