It takes a combination of many factors for a sperm to go in the vagina and be successful in getting pregnant. First of all this sperm must be of good quality because it must try to swim forward, if the sperm is not of good quality and cannot swim, then the chance of fighting for the egg is lost. Secondly, there must be an egg to combine with the sperm. If there is only sperm alone it is also impossible to form a fertilized egg. If the fallopian tube is blocked, the egg cannot be successfully transported and the sperm and egg will lose the chance to meet. If a fertilized egg is formed, it is halfway there. The fertilized egg divides continuously, but when it reaches 16 cells, it has to find a fertile place to lay its head. The development of the endometrium is extremely important at this time. If the endometrium is too thin to develop in the conditions needed for a fertilized egg, it will not be able to successfully implant and a biochemical pregnancy is common in clinical practice. The fertilized egg needs to have sufficient amounts of progesterone in the body at the time of implantation to allow the uterus to accept the fertilized egg and not reject it as a foreign population. In short, if all these conditions are met, conception will be successful. Any lack of one of them will end in failure.