Many factors can hinder the unification of egg and sperm cells, even when the egg is ripe and there are enough motile sperm cells. The most common cause is a blockage of the Fallopian tubes due to various illnesses, which in return doesn’t allow the fusion of the gametes. The blockage may occur when the walls of the Fallopian tubes get stuck together after infections, adhesion between ovaries and Fallopian tubes, when the Fallopian tubes are not present, after surgery or when the Fallopian tube is sealed after sterilisation. Sometimes the sperm cells become immobile in the cervical canal because of immunological processes.