| What can go wrong in conception?
What can go wrong?
The conception process is hardly foolproof. An ovary might not release an egg successfully. Or the egg might be flawed or damaged -- something that is more likely to occur the older the woman gets, or if she smokes, uses drugs, or is exposed to radiation or other phenomena that can damage immature eggs. A man might have trouble producing sperm, or his sperm may be misshapen or slow-moving. Damaged sperm or eggs may not be able to join successfully, or they may produce a fertilized egg that cannot survive the early stages of growth.
Other problems have to do with the organs and systems in the woman's body. A blocked or bent fallopian tube can prevent an egg from entering the uterus. Problems with the endometrium may keep a fertilized egg from implanting. And levels of hormones that are too high or low can throw off the entire cycle. Fortunately, medical treatment can now fix many of these problems.
Still, even if all the systems are in order, timing must also be right. If the sperm enter the uterus too long before the woman ovulates, they die before the egg arrives. If they come too late, they miss the chance to meet the egg in the fallopian tube.
Although fertilization can occur only when an egg is present, it's important to remember that not all women ovulate regularly. Sometimes an egg emerges earlier or later than usual. This is particularly true for younger women and for women with irregular periods. For that reason, some possibility always exists that a woman in her childbearing years might be fertile at any moment, even during her period.
For this reason, couples who do not want to have a baby should use contraception every time they have sex. The risk of conception at the beginning or end of a woman's cycle may be tiny, but it does exist. Couples that engage in natural family planning (the timing of sex to achieve or avoid pregnancy, based on physical signs of the woman's fertility) can reduce the odds of an unplanned birth by keeping close track of a woman's cycles and other signs over a long term. Even this system, however, can fail, simply due to the unpredictability of a woman's body | 
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