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Fetal Development & Physiology occurs in three stages: the preembryonic stage (fertilization to 2nd week), embryonic stage (end of 2nd week to 8th week), and fetal stage (end of 8th week to birth). The embryonic stage is the critical period of organ differentiation, making the embryo highly susceptible to teratogens (drugs, alcohol, or infections causing physical defects). Fetal support systems include amniotic fluid, which maintains temperature, cushions the fetus, and allows symmetric growth. The umbilical cord contains two arteries (carrying deoxygenated blo ...