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How does fertilization produce the single cell from which the human body develops?

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Fetal growth — human development Growth of the human fetus from the fourth month to the ninth month of pregnancy.

Fertilization is the process by which a male sperm and a female egg unite to create a single-celled zygote, the foundation of a new human life.12

The process begins when a sperm penetrates the egg’s outer layer, triggering surface changes that prevent other sperm from entering. Once inside, the sperm’s head transforms into a male pronucleus, while the egg completes its final maturation to form a female pronucleus. These two pronuclei meet within the egg’s cytoplasm, where their membranes dissolve and their genetic material combines.13

This union restores the full, diploid number of chromosomes—half from each parent—providing the essential genetic instructions for the individual. The resulting zygote also establishes the new individual's sex and activates the biological mechanisms required for further development. Shortly after, the zygote begins cleavage, a series of mitosis divisions that subdivide the single cell into smaller units called blastomeres, which serve as the building blocks for the developing organism.12