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Fertilization (English spelling: fertilisation) is what happens when a female's ovum (or "egg") joins together with a male's sperm and they form a zygote. Fertilization is also called conception.

In humans, biologists call a fertilized egg a zygote. It is called an embryo from 3–8 weeks (the embryonic period.) Then after 8 weeks, it is called a fetus.

If it continues to grow normally it can eventually become a baby.

People have different beliefs about when life starts:

  • Some people believe that life begins at conception.
  • Some people believe that life begins after the embryonic period (when the organs form.)
  • Some people believe that life begins when the fetus is able to live outside of the woman.
  • Some people believe that life begins after a baby is born.

This difference in what people believe has led to many arguments over abortion.

Fertilisation also occurs in animals. There are two types of fertilisation, internal and external. Internal fertilisation occurs in the female body, and external fertelisation occur outside of the body. All of the mammals and birds use internal fertilisation. Reptiles, amphibians and fish use external fertilisation.