Embryos at 4 to 8 Weeks: Rapid differentiation of cells and an astounding rate of growth characterize the first weeks of embryonic development. At 4 weeks, the embryo is the size of a grain of rice. Its heart has already begun to beat, and the early divisions of what will be the heart's four chambers are apparent. At 6 weeks, the embryo may be half an inch (10-14 mm) long and is starting to acquire a human face, although it is impossible to differentiate male from female embryos at this stage. An 8-week-old embryo may measure over an inch (28-30 mm) in length, and all of the body's parts-cells, tissues, organs, systems-have been differentiated.
Prenatal or antenatal development is the process in which a human 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 process of embryogenesis, (the early stages of prenatal development) begins. By the end of the tenth week of gestational age the embryo has acquired its basic form and the next period is that of fetal development where the organs become fully developed. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age.
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