When we look at all the events that must happen in perfect synchronization and all the difficulties that sperm and eggs must overcome in order to find and produce an embryo, we fully realize that the creation of a baby baby is a real feat.
In order to reach the egg, the sperm have to go through a marathon that starts in the vagina, runs through the entire uterus, continues along the entire length of the tube until they find the egg in the farthest part. It is a long road (the approximate equivalent for a man is 40 km) and full of obstacles. The entrance to the uterus, the cervix, presents alabyrinth of passages and throughout the journey the woman’s immune system attacks and destroys countless sperm. Only a few, of the millions that are deposited and n the vagina they manage to reach the egg.
A second difficulty arises for the egg (oocyte) to reach the tube. When ovulated, the oocyte can fall freely into the uterine cavity or be picked up by the fallopian tube. Only 4 out of 10 oocytes manage to be rescued by the tube. Since ovulation occurs once a month, this means that in 10 months there are only 4 months in which the oocyte is trapped by the tube and can be fertilized and 6 months in which the oocyte is lost in the uterine cavity and pregnancy is not possible, even if sexual intercourse is had on fertile days.
Once the sperm find the oocyte, they must cross one more barrier. The oocyte has a hard, thick covering called the zona pellucida and 
The sperm must be able to attach to and penetrate the zona pellucida before they can fertilize the oocyte. About 50 thousand sperm are required in a drop of embryonic culture to fertilize an oocyte.
Finally, when the embryo arrives in the uterine cavity, it must choose a place 
suitable for attaching and “taking root” by connecting to the mother’s circulatory system and thus obtaining the necessary nutrients for her nutrition and growth. This process is not always successful. Out of every 10 embryos that reach the uterus, only 2 or 3 are successfully implanted and 7 or 8 are lost without the woman knowing that she had an embryo in her uterus.
In the end, the probability of pregnancy, for a young couple, during their first year of search is only 20% per month. Sometimes it may take 12 months to eventually get pregnant.
