Image Caption : 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 development is the process in which an embryo and later fetus develops during gestation. Prenatal development starts with fertilization the first stage in embryogenesis which continues in fetal development until birth.
In human pregnancy, prenatal development, also known as antenatal development, is the development of the embryo following fertilization, and continued as fetal development. By the end of the tenth week of gestational age the embryo has acquired its basic form and is referred to as a fetus. The next period is that of fetal development where many 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.
In other animals the very early stages of embryogenesis are the same as those in humans. In later stages, development across all taxa of animals and the length of gestation vary.
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