You are probably thinking of the Shettles Method which is based on the premise that sperm carrying the X and Y chromosomes have different characteristics and that you can influence the reproductive environment to favor one or the other. While there is some controversy surrounding both the credibility and the morality of the method, here's how the theory goes:
It is the sperm that fertilizes the egg that will determine the sex of your baby. Some sperm carry the X-chromosome and some sperm carry the Y-chromosome. If an X-chromosome-carrying sperm fertilizes the egg, the baby will be a girl. If a Y-chromosome-carrying sperm fertilizes the egg, the baby will be a boy.
The idea behind the Shettles Method of sex selection is based on the premise that the X and Y chromosome carrying sperm have different characteristics and that under different circumstances either X- or Y-chromosome-carrying sperm will be more likely to fertilize the egg.
The Y-chromosome-carrying sperm are said to:
be faster and smaller than the X-chromosome carrying sperm.
die faster than the X-chromosome-carrying sperm.
The X-chromosome-carrying sperm are said to:
be slower than the Y-chromosome carrying sperm.
be better able to withstand the acidic cervical environment before fertile cervical fluid is produced.
Based on these premises, according to Shettles, you can time intercourse, choose a sexual position that favors conception of your preferred sex and influence the reproductive environment to increase the likelihood of conceiving your preferred sex. These are the suggestions from Dr. Shettles:
To Get A Boy (According to Shettles):
Time intercourse as close to ovulation as possible: The idea is that since the Y-chromosome sperm are faster than the X-chromosome sperm, there will be more Y-chromosome sperm who reach the egg, making it more likely that a Y-chromosome carrying sperm will fertilize the egg.
Men avoid tight clothes: heat kills off both types of sperm, but will kill off the less protected, smaller Y-chromosome sperm faster, according to Shettles.
Women have an orgasm: According to Shettles, female orgasm increases the alkaline secretions in the vagina that are favorable to the Y-chromosome carrying sperm. Shettles recommends having an orgasm before or at the same time as the male partner.
To Get a Girl (According to Shettles):
Have intercourse 2-3 days before ovulation and avoid intercourse just before ovulation until 2 days after ovulation and when you have peak cervical fluid: The idea is that when you have sex a few days before ovulation, only the X-chromosome “girl sperm” will be left in the female reproductive tract waiting to fertilize the egg when it is released.
Have intercourse with shallow penetration: Shettles recommends “missionary position”or any position that will deposit the sperm slightly away from the cervix, giving advantage to the longer living, but slower X-chromosome-carrying sperm.
Women avoid orgasm: Shettles suggests women avoid orgasm because it makes the vaginal environment more alkaline, and less acidic and is disadvantageous to the X-chromosome “girl sperm”.
If you are taking longer than expected to conceive, it is generally not recommended to attempt any kind of sex selection as it can increase the time it takes to conceive.
See also: Review of scientific articles about intercourse timing and gender selection.