Dr. Landum Shettles, author of the famed book Choose the Sex of Your Baby, devised a method, namely Shettles method, of naturally increasing your odds of concieving the gender of your choice.
Genetic science tells us that male sperm is genetically encoded to bear either an X chromosome or Y chromosome. Barring some very rare genetic disorders, a woman consistenly contributes only an X chromosome. When a sperm carrying a Y chromosome fertilizes an egg, it will become an XY combination and will produce male offspring. Alternatively, when an egg is fertilized with an X-bearing sperm, the combination is XX resulting in a female.
Dr. Shettles concluded in his research that X and Y-bearing sperm have different characteristics and will perform differently in identical conditions. And he bases his method upon the premises that X-bearing sperm are slower moving but live longer than the faster swimming, shorter lived Y-bearing sperm. If all conditions are equal and an egg were waiting to be fertilized, he felt that the faster Y-bearing sperm would reach it first.
-"To increase the probablility of female offspring, insemination should occur 2 or more days before ovulation, with acidic douches, using the missionary position, and without female orgasm."
-"To increase the probability of male offspring, insemination should occur as close to the time of ovulation as possible, with non-acidic douches, using the rear-entry position, and with female orgasm."
Do take note that he only recommended douches in the first edition of his book, it was later found that this could increase risks of infections. The Ph balance of the vaginal environment changes throughout a woman's cycle to both protect her from infection and to promote pregnancy. Altering this natural balance can certainly open the door to danger.
Most all preselection methods will ask that you track your cycles for at least 3 months prior to attempting. The key here is to pinpoint the time of ovulation so that insemination can occur in the time of higher probability depending on the gender you desire.
There are several ways to track ovulation. First of all, in an average 28 day cycle, ovulation should typically occur at or around day 14.
A surge in Luteinizing Hormone occurs when there is rise in progesterone, which typically occurs within 24-36 hours of ovulation. Over-the-counter Ovulation Predictor Kits come with urine testing strips that detect this hormone. These kits sell at any drug store or major retailer for anywhere from about $15 to $40 for a digital.
A woman's cervix can give very accurate information as to where she is at in her cycle. The cervix produces a mucus that changes throughout the month, the changes are easily indentifiable and give direct evidence as to how fertile a woman is at any given time. The positioning of the cervix also yields clues to a woman's fertility, it tends to be higher, softer, and slightly more open the closer she gets to ovulation.
Basal Body temperature(BBT) is another way of tracking ovulation. A woman's body temperature will show characteristic peaks and valleys according to the time of ovulation. For this, it is important to buy a special Basal thermometer(wal-mart has them for 9 bucks) and take it very first thing in the morning, before any physical activity, not even getting out of bed. BBT charts typically come with the thermometer, but can also be printed online.
Glucose test strips (about 7 bucks at any pharmacy) can be used to test the amount of glucose present in the cervical mucus, as the level of glucose becomes higher closer to ovulation.
Accurately tracking your cycle is very important no matter what method you choose to follow.
There are other things that are suggested that increase probability. It is suggested that a more acidic vaginal environment can be fatal to Y sperm, favoring X sperm, and a more basic environment favors survival of Y sperm. It is also believed that a woman's orgasm causes the release of an alkaline fluid. So it is suggested that for a boy, a woman should orgasm before or at the same time as the man. And for a girl, a woman should avoid orgasm altogether.
When trying for a boy, deeper penetration at the time of insemination can reduce the sperm's contact with the acidic vaginal environment, so a rear-entry position is recommended during intercourse. And for a girl, a more shallow penetration can expose the sperm to this environment and reduce the number of Y sperm that make it through the cervix.
Again, if you're interested, go read the book so that you get the full information. And personally, I wouldn't stop there, as there are many critics of Shettles method, and I think it's important to find what you believe to be true for yourself, not just because someone says it's true.