shettles developed a method to maximize the probability of having a baby of the sex of the parents' choice. using his "shettles method," couples who wanted to have a male baby should time intercourse as close as possible to ovulation to allow the faster y-bearing sperm to reach the egg first. couples desiring a female should time intercourse to take place about three days prior to ovulation, when the ph of the vagina is more acidic and thus more hostile to the faster but less bulky y-bearing sperm, and therefore favoring the bulkier x-bearing sperm on a small level.