Parents have been employing pregnancy gender selection for thousands of years but it is only recently that the scientific knowledge about the human reproductive system has allowed us to fully understand the biology that creates boys and girls.
In simple terms we now know that there are both boy (containing a Y chromosome) and girl sperm (containing X chromosomes) and which ever one fertilises the mother's egg decides the baby's gender.
In the early 1960's Landrum Brewer Shettles, the leading fertilisation expert at Columbia University produced a book explaining his methods for selecting a baby boy or girl.
Pregnancy selection by his methods have remained the best, most respected and most used methods for conceiving a baby with the gender of your choice.
Shettles' methods are all based on a discovery that the boy and girl sperms have different characteristics.
The boy sperms being smaller and faster but short lived while the girl sperms are stronger, live longer but are slower swimmers.
Further more each sperm type survives better at different ph levels.
Shettles therefore explains how you can 'adjust' the environment to better suit one sperm type or the other during conception and so dramatically increase the chances of conceiving a boy or girl.
Pregnancy selection therefore gives you the baby gender of your choice.
In some tests this advantage has been shown to be as much as nearly 95%.
Of course this is a very simple overview of pregnancy gender selection.
The details and practices required to ensure the best chances of conceiving a baby boy or girl need to be well understood and exact.
To that end you will need to study one of the more modern adaptations of Shettles' book.