Prenatal development is the process in which an immature individual develops before birth. For instance, mammalian prenatal development is the process where an embryo or fetus (or foetus) gestates during pregnancy, from fertilization until birth.
Prenatal development starts with fertilization, which marks the beginning of embryogenesis. Embryogenesis, in turn, continues in fetal development, which lasts until birth.
Fertilisation or fertilization (also known as conception, fecundation and syngamy), is fusion of gametes to form a new organism of the same species. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation.
Embryogenesis is the process by which the embryo is formed and develops. It starts with the fertilization of the ovum, egg, which, after fertilization, is then called a zygote. The zygote undergoes rapid mitotic divisions, the formation of two exact genetic replicates of the original cell, with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo. It occurs in both animal and plant development.
In mammals and other viviparous vertebrate, the embryonic stage is succeeded by the fetal stage. Often, however, the terms fetal development and the whole process of prenatal development are used in a similar sense.