Found this on a website about smep
It quite interesting so thought i would share, don't know how true it is though, sorry if this has been put up before or if you knew this.
Many books tell you that sperm can last for 5 days and the egg for 24 hours. While this is technically true on the very long end (and something to follow if you trying to NOT get pregnant), most sperm will only last about two hours if you do not have fertile-quality cervical mucus for it to swim in. The sperm will struggle to swim up to your uterus, use all its reserves, and not make it. The egg typically lives only about 12 hours, so it cannot wait for long. You can now see how important that cervical mucus is! You will never get pregnant with sperm living two hours and an egg only twelve. This information is really just to make you feel better if you've been trying a long time and all your infertility testing came back normal. If the Deanna-plan does not work and you are faithful to it for three months, take a dose of plain Robitussin cough syrup (or any cough medicine that says "expectorant" and NOT "antihistamine") each day (preferably a few hours prior to "trying") starting around Day 10 until the day after your ovulation predictor goes off. It should help make all the mucus in your body runnier, including that produced by your cervix. (Oh the gruesome details required in baby-making!) The sperm in the runnier mucus will live about two days, and will be up there and ready for the 12-hour life of the egg. A NOTE ABOUT CLOMID: Clomid causes cervical mucus to dry up in 25% of the women who take it. If you notice your mucus is not plentiful as it was before taking this drug, take the Robitussin and call your doctor to make sure your really need the Clomid. If you are ovulating on your own and do not have a documented luteal phase defect, you most likely do not need it.
"Trying" too often can actually do more harm than good. Do not try every night! You will get exhausted and sore, and your mucus--both for fertility and for lubrication--will dry up, and you will stop trying too early in the month or miss an important day. Every other day is absolutely sufficient, with three nights in a row during peak time sealing the deal.
Don't worry about stress! Regular old worries about getting pregnant, and if you will ever have children, are perfectly normal and do NOT affect your fertility. Ignore those people who tell you just to relax and stop thinking about it. This is not their problem! The only thing that could actually affect you is serious stress, like moving to a new house, losing your job, family deaths, and other things that make you physically ill or depressed. This can delay your ovulation, or make you not ovulate in a cycle, since you will produce an excess amount of the stress byproduct called prolactin. It will not affect you for long, and the next month you should come back and be normal again.
An early period is not an indication of an early miscarriage, even if you know you timed your trying perfectly. Usually it means that the egg was not fertilizable, and so progesterone was not adequately produced. This shortens your cycle. Sometimes eggs simply don't develop properly during the ovulation process. It is usually a one-month problem. If you are regularly seeing that fewer than 10 days are passing between ovulation and your period, however, it's time to be tested for a luteal phase defect. You can read more about that under hormone causes of miscarriage.
Here;s the web page anyway
http://www.pregnancyloss.info/sperm_meets_egg_plan.htm 
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