If the ova are fertilized inside the female’s body, this is known as
internal fertilization. One advantage of internal fertilization is that the eggs are better-protected than are those of animals that practice external fertilization – up until the moment of fertilization, at least. Another advantage of internal fertilization is that the sperm can more easily fertilize the ova. Probably not coincidentally, animals that practice internal fertilization tend to produce fewer eggs than do closely-related species with external fertilization. If fertilization is internal, it isn’t such a random process, and the chances of an egg being successfully fertilized are higher.
In many species, after the egg is fertilized, the female secretes a protective shell around it that helps protect the developing embryo inside. This means that the eggs can then be deposited in relatively dry environments. Obviously, this is possible only if fertilization is internal, since the egg can’t be fertilized
after a shell is secreted around it.
In some species, such as most mammals, the embryo remains inside the mother’s body as it develops. Obviously, in such species, fertilization
must be internal.

In birds (Phylum Chordata), fertilization occurs inside the female’s body.