To become pregnant, a woman must have intercourse during or near the time when she is ovulating. Her fertile period lasts from the time the egg is released from the follicle until about 24 hours later, when the egg is no longer viable. Even if intercourse is perfectly timed, there are many reasons why a pregnancy won't occur.
Missed the Target
Even with perfect timing, the sperm still must reach their goal--the egg. It is possible that the sperm simply never made it to their goal.
To pass through the cervix into the uterus, sperm must move from the ejaculate into to the cervical mucus. This mucus acts as a reservoir for sperm; it can also act as a barrier that may be difficult for sperm to pass through. If the cervical mucus is not abundant and does not have the right texture or consistency, becoming pregnant can be difficult even when a woman is ovulating.
The quality of a woman's eggs can affect whether or not she will become pregnant. Younger women in their 20s and 30s should have a reserve of good eggs; as women age, the quality of their eggs declines. Even if an egg was released and sperm was there to fertilize it, the resulting blatocyst may not have been able to develop into an embryo.
According to the American Pregnancy Association, male infertility causes as many as half of infertility cases. A man's sperm may be low in number, slow to move or have the wrong shape. These factors may make it difficult for a sperm to reach the egg.
Poor Uterine Conditions
After fertilization, the egg must travel to the uterus, where it joins with the uterine lining. If the lining is not thick enough to support an embryo or does not accomplish this quickly enough, progesterone levels in the body will drop and menstruation will occur, even though fertilization was achieved.