Actually the egg in a way chooses the sperm. The egg releases chemical signals that guide sperm toward it. Different eggs may produce slightly different chemical cues that can attract some sperm more than others. Some studies show that eggs may favor sperm from genetically compatible males particularly those with immune system genes that differ from the female’s, which can lead to healthier children. Even when multiple sperm reach the egg, the outer layer can block entry to sperm that don’t bind correctly or are less compatible at the molecular level.