搜出来，这个叫Shettles method
但又说很难去验证。。。

The idea that you can naturally control your baby’s sex comes from a theory developed by Dr. Landrum B. Shettles. The basis for the Shettles method starts with the fact that X-chromosome sperm (female) are on average slightly larger and thus slower moving than Y-chromosome sperm (male). And X-chromosome sperm live longer. So, according to the theory, if a woman can predict with confidence the day she will ovulate or release an egg, then have sex 3 or 4 days beforehand and not again until well after ovulation, there is a higher likelihood of producing a female child. This is because the longer living female sperm may still be hanging around 3 to 4 days later.

【 在 worst 的大作中提到: 】
: 据说能活两三天呢
: 如果卵子后到
: 生出来极大可能是女孩
: ...................
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FROM 24.203.11.*