哥伦比亚大学教授Landrum B. Shettles和David M. Rorvik专门研究已婚夫妇应当如何通过ML方式和ML时间的选择来决定生男生女，专著《Howto Choose the sex of Your Baby》要点摘录：

 “The male spermis lighter, faster, and round headed. It is also more fragile and subject to heat, toxins, and other risk factors. The female sperm is slower but hardier with a more oval head.”

综合书中其他描述：精子可以分为男精子和女精子，各有其特性。简略而形象的说来，男精子就是缩微版本的男人，而女精子就是缩微版本的女人。男人和女人有何差别呢？男人外表强大、有力，实则不能久经考验，而女人长寿、生命力顽强持久。男精子活跃、暴发力强、游动速度快、气势汹汹，但是环境适应能力差，要么在某种性质的体内环境下存活时间短，要么在某种性质的体内环境下忽然就蔫了、游不动了，在长时竞争与长途竞争中多半要输，看上去很好很强大，但是却往往很容易夭折掉，也多见其半途而废，女精子反之。

如此推断，可以得出一些较为确定的分析结果，比如通过排卵当日ML来缩短竞争时间则更有利于男精子竞争获胜；比如男方保持充沛的精力厚积薄发、女方需要得到更大满足则可缩短竞争路程，同样有利于男精子竞争获胜。当然书中说的更为详尽，只能引述其大概，恕不能一一例举。

“TIMING is the single most important factor in sex selection. ”

时间是决定后代性别的最重要因素。

“Time intercourse for the day of ovulation, and if possible, within 12 hours. Even if intercourse takes place several hours before ovulation happens, but with a day of it, the odds are infavor of the male producing sperm.”

排卵日ML，最好于排卵12小时以内。排卵数小时前ML，或在一天内，有利于男精子竞争获胜。

“Your best chances for having a boy are in the 12 hours before ovulation occurs. But chances are still significant 24 hours before ovulation.”

如果想要男孩，排卵前12小时ML是最好的，排卵前24小时也很好。

“Twelve hours before ovulation is ideal but anytime 24hours before ovulation should increase chances for a boy.”

排卵前12小时ML，或排卵前24小时的任何时候ML，都有利于男孩的出生。

“When trying for a girl, have sex daily from the time bleeding ends until the cut off day – four,three, or two days before ovulation. Abstain from sex completely until a few days after ovulation has passed because intercourse may stimulate female secretions that favor male sperm.”

如果想要女孩，从流血结束那天开始，直到排卵的四天前、三天前或两天前的截止日，每日ML。而且排卵后应当完全禁止ML，因为ML会刺激有利于男精子的雌性激素的分泌，

（When trying for a girl ）Intercourse Recommendations: Dr. Shettles says the “missionary position” makes it less likely the male sperm will be deposited near the cervix. Shallow penetration will deposit the sperm in the more acidic vaginal canal, favoring the female sperm. Finally, the woman should avoid orgasm, if possible, so the alkaline secretions that favor male sperm will not emitted, nor will the orgasmic muscle contractions that help propel the sperm towards the egg.

想要女孩的话，关于ML的建议：男上女下不利于男精子沉积于子宫颈。肤浅的渗透将精子储存于更为酸性的阴道环境，有利于女精子。最后一点要注意的是，应避免让女方爽，这样有利于男精子的碱性分泌物就不会激发，而且将精子推向卵子的肌肉收缩也不会发生。让女人爽，有利于生男孩，让女人不爽，有利于生女孩。

另：因书中术语太多，阅读难度较高，误解的后果可能适得其反，所以其他方面的注意事项以及排卵日的确定方法不再介绍，请百度。