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2. Chapter 21 Growth and Development

3. PRENATAL PERIOD
Prenatal period begins at conception and continues until birth (about 39 weeks)
Science of fetal growth and development called embryology
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4. PRENATAL PERIOD Fertilization to implantation requires about 10 days
Fertilization normally occurs in outer third of oviduct (Figure 21-2)
Fertilized ovum called a zygote; zygote is genetically complete—all that is needed for expression of hereditary traits is time and nourishment
After 3 days of cell division, the zygote has developed into a solid cell mass called a morula
Continued cell divisions of the morula produce a hollow ball of cells called a blastocyst
Blastocyst implants in the uterine wall about 10 days after fertilization
Blastocyst forms the amniotic cavity and chorion of the placenta (Figure 21-4)
Placenta provides for exchange of nutrients between the mother and fetus
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7. PRENATAL PERIOD Periods of development
Length of pregnancy, or gestation period, is about 39 weeks
Embryonic phase extends from the third week after fertilization to the end of week 8 of gestation
Fetal phase extends from week 8 to week 39 of gestation
All organ systems are formed and functioning by month 4 of gestation (Figure 21-6)
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