Blastoff9600

Actually it isnt a myth. Male sperm swim faster and have a shorter life span than female sperm which swim slower but live longer. Positions dont really play a role in determining sex. Though when you have sex can increase your odds of one sex over another. A great book that explains this a bit better is Taking Charge Of Your Fertility by Toni Weschler and in her book there are a list of some other books that go into greater detail on increasing your odds for one sex over another.
When you are trying for a girl you need to have sex up to four days before O and then stop for that cycle. If you dont get pregnant that cycle then have sex up to 3 days before O and then stop(or use protection) after that. If that doesnt work for that cycle then the next cycle have sex up to 2 days before O and then stop for that cycle. The closer you are to O the higher the chances of a boy. The day before and the day of O you chances of a boy are higher. But it isnt set in stone and you could still wind up pregnant with a boy no matter what you try.

Also you cant really go by you past cycles on predicting O because your body is still adjusting to being off birth control. Also your cycles could have changed from your past ones so what day you Oed in tha past may not be the same. Reading that book I mentioned could help you.
Also just to let you know dont count on the fact that your family is fertile to base thatyou are fertile. I ought to know that doesnt work the way one wants it too. Most the women in my family can get pregnant from their spouse/partner just looking at them. My own mother had 10 kids counting me. Birth control for most the women in my family also fails because they are so fertile. But as you can see by my sig I have fertility issues that occurred after I was on Depo. Family history of good fertility doesnt mean you will be so lucky. Not trying to be mean just saying things can happen that you dont expect.

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Married 3/25/95
DS 13 yr
DS 8 yr (3+ yrs ttc)
TTC #3 since 01/02