It is a violent, microscopic competition. Thousands of sperm reach the Fallopian tube. Only a handful touch the egg. They surround it. The race begins.
Each sperm uses its head, specifically the acrosome, to dump enzymes onto the egg’s outer membrane. Think of it as a jelly-like shield. The enzymes eat away at it. One sperm has to get inside. Once that single entry happens, everything changes instantly.
The egg’s cell membrane shifts its electrical charge. This signal triggers cortical granules—tiny sacs just under the surface—to burst open. Their contents swell up. This swelling pushes all the other sperm away. It is a hard lock. A biological firewall.
The cortical reaction ensures that only one sperm fertilizes the egg.
The rest die within 48 hours. They never stand a chance.
The fertilized egg is now a zygote. The electrical spike causes one final nuclear division. It forms a pronucleus with a single set of genetic instructions. The sperm’s pronucleus meets it. They merge. Cell division starts immediately.
No pause. No hesitation.
The zygote travels down the Fallopian tube. It divides rapidly. By day four, it has about 100 cells. It is now called a blastocyst.
It floats near the uterine lining for two days. Then, it burrows. Implantation happens around day six. This is the moment pregnancy officially begins.
The blastocyst stays in the uterus. It grows. The uterus stretches. Eventually, it reaches the size of a basketball. The cycle completes.
