It is particularly important to understand that for the human species, neither all the spermatozoa, nor all the eggs that are produced and released are fertile. Accordingly it is not guaranteed that all the fertilized eggs will manage to arrive in the uterus, or even if they do manage to arrive in the uterus it is not guaranteed that they will be successfully implanted in the endometrial cavity and eventually lead to a healthy child live birth.
In other words, an egg, a sperm or an embryo, are just possibilities of conception and not certainty. Due to all these limitations our species is the most difficult to reproduce in the animal kingdom and thanks to them nature ensures that reproduction will occur only when it is absolutely sure about the result produced.
