The pH of a woman's vagina is also very important. An acidic environment favors girls, since it will kill the weaker male sperm first and leave a greater number of X sperm to fertilize the egg. So, if you are trying for a boy, a good time for intercourse is when the vaginal balance changes to alkaline—on the day you ovulate—otherwise the male sperm will either slow down or die in an acidic environment.
Position of intercourse and depth of penetration are also important, partly because of pH. The closer to the entrance of the vagina, the more acidic the woman's tract is. Deep penetration places the sperm closer to the egg and gives the faster male sperm a head start. Shettles recommends the missionary position for girls and deeper penetration positions such as rear entry for conceiving boys.
Obviously a low sperm count to begin with doesn't favor either sex, but a high sperm count favors boys. Shettles recommends that when trying for a boy, the man wear loose underwear because the testicles need cooler temperatures for sperm to survive and thus the weaker Y (male) sperm will benefit from this more than the stronger X (female) sperm.
For a girl, it is suggested that the man have a hot bath immediately before intercourse to give the X (female) sperm an advantage (by killing off the weaker male sperm).
Food for Thought
It is thought that minerals in your diet affect the metabolism and environment of the egg and therefore make it more attractive to sperm of one particular sex. If you want a boy, eat plenty of potassium-rich foods such as meat, bananas, apricots, and celery. If you want a girl, lots of magnesium-rich foods such as nuts, soya beans, and leafy green vegetables should do the trick.
While some couples swear by gender selection methods, others say that there is no certain way to choose the sex of your baby. No matter which theory you believe, there is one definite: you and your partner will have fun trying out the various methods!