Fertilization, the union of a sperm and egg, occurs once a month because ovulation, the release of a mature egg, happens only once per menstrual cycle. This cycle typically lasts around 28 days. Only one egg is released during each cycle, and it can only be fertilized within a limited timeframe (usually within 24 hours).
Here’s a more detailed explanation:
Ovulation:
During the menstrual cycle, one of the ovaries releases a mature egg (ovum) in a process called ovulation. This usually happens around the 14th day of the cycle.
Sperm Journey:
During sexual intercourse, millions of sperm are released, and they travel through the female reproductive tract.
Fertilization:
If the egg is present in the fallopian tube, a single sperm can penetrate the egg’s outer layer and fuse with its nucleus.
Zygote Formation:
This fusion creates a fertilized egg, called a zygote.
Implantation:
The zygote then travels down the fallopian tube and into the uterus, where it can implant in the uterine lining if fertilization occurred.
Since only one egg is released per cycle, and it is only viable for fertilization within a short window, fertilization can only occur once during each menstrual cycle.