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Prenatal development is the process in which an embryo or fetus (or  foetus) gestates during pregnancy, from fertilization until birth. Often, the terms fetal development, foetal development, or embryology are used in a similar sense. After fertilization the embryogenesis starts. In humans, when embryogenesis finishes, by the end of the 10th week of gestational age, the precursors of all the major organs of the  body have been created. Therefore, the following period, the fetal period, is described  both topically on one han d, i.e. by organ, and strictly chronologically on the other, by a list of major occurrences by weeks of gestational age. Contents [hide] 1 Fertilization 2 Embryonic period o 2.1 Changes by weeks of gestational age 3 Fetal period o 3.1 By organ o 3.2 Changes by weeks of gestational age 4 References 5 External links [edit] Fertilization Main article: Human fertilization A sperm fertilizing an ovum When semen is deposited in the vagina, the spermatozoa travel through the cervix and  body of the uterus and into the Fallopian tubes. Fertilization of the ovum (egg cell ) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its genetic material  (DNA ). The resulting combination is called a zygote. The term "conception" refers variably to either fertilization or to formation of the conceptus after uterine implantation, and this terminology is controversial.

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