The path of the sperm to the egg is not easy, quite the contrary. Therefore, it is necessary to have a good number of motile spermatozoa because only one, the fastest and most skilful, will reach the egg to fertilize it.
Along the way, the sperm have to cross a series of barriers and obstacles. One of them is the pH of the vagina, as it is quite acidic. In addition, the woman generates an immunological reaction to the sperm, detecting them as foreign.
On the other hand, a woman's cervical mucus varies according to the time of her cycle. During the ovulation phase, the cervical mucus is more liquid and allows sperm to pass through. In contrast, during the luteal or pre-ovulatory phase, the cervical mucus is thick. This makes it difficult for sperm to pass through.