Sperm take as little as 30 minutes to six days after sexual intercourse to get to the Fallopian tubes of the uterus. They are aided in their upward movement by contractions of the muscle of the uterus and tubes. Instead of going all the way to meet the egg, they tend to sit in the tubes and wait for the egg to be released from the ovary in ovulation. The sperm then start swimming again when the egg is released, in the final leg of their journey to be the first to fertilize it.
The classic picture of sperm swimming strongly to the Fallopian tube to be the first to get to the egg is not quite accurate. Instead, the sperm swims to the Fallopian tube and then spends about seven hours undergoing a process called capacitation. This process involves the sperm removing some of its protein coat so it is ready to get through the corona radiata.
Corona Radiata Penetrance
Capacitated sperm can easily get through the corona radiata, the outer layer of the egg. While only one sperm can eventually fertilize the egg other sperm also try to get through the corona radiata by breaking it down with enzymes. This helps weaken the layer and makes it easier for the triumphant sperm to get through.
Zona Pellucida Penetration
The next layer of the egg that the sperm has to get through is the zona pellucida. The process of getting through this is more complicated than getting through the corona radiata. First, the sperm has to bind to the zona pellucida. This binding triggers the sperm to release enzymes that can break down the zona pellucida at the point of binding. This process is called the acrosomal reaction, and it involves enzymes. It would be detrimental to fertilization to allow more than one sperm to reach the interior of the egg, as this can lead to abnormal DNA mixing and replication. Occasionally this happens, but the embryo generally dies. Therefore, when the successful sperm gets through the zona pellucida to the plasma membrane of the egg inside, this triggers the zona pellucida to become resistant to other sperm.
Fusion of the Membranes
The sperm membrane fuses to the egg membrane, and the insides of the sperm enters the insides of the egg. The egg membrane then becomes impenetrable to other sperm, just in case, and the egg undergoes its second meiotic division, in which the egg splits in two, with each division retaining half the original set of DNA. Only one division remains as the egg, which contains a pronucleus with DNA in it. The other division is called a polar body and degenerates. The sperm moves to the egg pronucleus, and the tail falls off. The sperm pronucleus then merges with the egg pronucleus and the resultant fertilized egg begins to replicate its DNA and begins dividing into an embryo. The embryo then travels down into the body of the uterus and can implant into the uterine wall about six days later. If implantation occurs, then the woman becomes pregnant, but it may not, which becomes an early miscarriage.