What Happens After the Sperm Reaches the Egg?

During human reproduction, a female generally releases only one egg per month, while a male typically releases millions of sperm during one ejaculation. These sperm fight an upstream battle, literally, to get to the egg to fertilize it during the 24-hour period that it is viable to create a pregnancy. Still, reaching the egg is only half the battle. What happens next is the most important part if you’re trying to get pregnant.

Egg Membrane Penetration

Of the millions of sperm that are released during sex, only a few hundred will actually reach the egg. At that point the sperm begin to whip their tails frantically. This helps them to penetrate the eggs outer membrane, which is softened by a chemical on the head of each sperm that allows the sperm to burrow through the membrane.

Egg Membrane Protection

Once a single sperm cell successfully penetrates the egg’s membrane, the membrane immediately transforms into an impenetrable barrier to prevent other sperm from getting through. This portion of the fertilization process happens in the fallopian tube, usually in the widest portion near the uterus 1.


As the egg and sperm chromosomes combine, the egg is officially fertilized, creating a zygote. The combined chromosomes begin to divide into new cells, forming an embryo. This is sometimes also referred to as a blastocyst, a small ball of about 100 cells, until implantation.


A pregnancy isn’t fully viable until the embryo created during fertilization implants in the uterine lining. In order for implantation to occur, the embryo must break back through the membrane to burrow into the uterine lining, a process called “embryo hatching.” It can take the embryo anywhere from three to seven days to travel from the fallopian tube to the uterus and hatch before implanting into the uterus to form a pregnancy.