A women is only fertile for about two days a month as the egg makes its way down her fallopian tubes. Sperm can survive to fertilize the egg for about two days once they are inside her vagina and uterus. So, with any random act of sex a woman's chance of being fertile is almost 12%. Further issues come from the fact that when the egg and sperm are present together the egg rejects almost 3/4s of the sperm around it preventing a pregnancy. So, our 12% chance drops to a little less than four percent. This is why it can be so hard for some women to get pregnant, while others get pregnant the first time they have unprotected sex.
More issues can be caused by the sperm can get lost and never finding the egg, and the egg is not the passive recipient that was thought in the past, it actually has the ability to turn down or refuse entry to a sperm. So, just as you may turn a guy down, the egg may tell a sperm to get lost. It can be assumed this decision is made based on some level of genetic or chemical compatibility we don't fully understand yet. On the other hand and egg can be fertilized and then rejected by your body, many women also find their first pregnancies is rejected and others are miscarried or spontaneously aborted early on without our even knowing an egg was fertilized.
Keep in mind the figures above are for a completely random act of sex, but because most people have work and a schedule most women have sex in the early morning, evening or late at night and more often on the weekends then the week days. It is also more common on weekends than during the week. Women also tend to avoid sex during their periods (though this is a great time for shower sex as it helps with the cramping). Also a woman's cycles will make them more responsive or interested in sex when fertile than when they are not. The same hormones that control ovulation and the onset of our periods also affect a woman's desire, this is part of a woman's natural sex drive and even plays a key roll in methods of getting, or avoiding pregnancy.
