Fertilization occurs when a sperm and egg combine to form a zygote, a single cell. Over the first two weeks, the zygote becomes a blastocyst that implants in the uterine wall and separates into three germ layers - ectoderm, endoderm, and mesoderm. From weeks 3 to 8, known as the embryonic period, organogenesis occurs and 95% of major organs develop. During the fetal period from week 9 until birth, the fetus continues growing and developing, becoming viable outside the womb at around week 24.

1. Prenatal
Development
Chapter 3

4. Fertilization
Fertilization—process that occurs in the
fallopian tubes when the sperm and ova
combine to form a zygote
Zygote—a single cell organism that results
from the fertilization of an egg cell by a
sperm; the fertilized egg
Gametes – sexual reproduction cells
Ova—female reproductive cell
Sperm—male reproductive cell

6. The Germinal Period: 0 – 2 Weeks
Blastocyst—the term given to the zygote at
about day 5 or 6 postfertilization

7. o Includes the separation of
the three germ layers of
somatic cells
o Ends with the implantation
of the gastrula into the
uterine wall
Germinal period begins with the fertilization of the egg

8. Ectoderm – nervous system
(including the brain and spine),
skin, nails, and hair, as well as the
salivary, pituitary, and mammary
glands
Endoderm –thyroid gland,
bladder, lungs, and digestive
system
Mesoderm –heart, circulatory and
lymph systems, connective tissue,
muscle, and bones

9. The Period of the Embryo: 3 to 8 Weeks
Embryonic period –beginning of week 3
and continuing through week 8;
organogenesis takes place
Organogenesis – development of major
organ systems during the embryonic period;
by the end of week 8, 95% of major organs
are complete

11. Third week: The neural plate forms and
becomes the brain and neural tube (spinal
cord). Neurogenesis, the production of
neurons begins. The heart chambers and
blood vessels develop.
Eighth week: The embryo appears more
human, the inner and middle ear develop, and
the embryo is the size of a grape.

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Periods of Prenatal Development
The Embryo and Its
Supporting Structures

14. The Amnion
• The amnion is one of the life
support systems that develops
from the trophoblast
• Develops from the fertilized
egg not from the mothers body

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Periods of Prenatal Development
Formation of the
Neural Tube and
Neural Tube
Defects

16. The Period of the Fetus: 9 Weeks to
Birth
The period of prenatal development that begins after organogenesis is
95% complete at about week 9 and continues until birth.

17. Fetal Development
By 24 weeks: Fat begins to accumulate. Bone marrow begins
producing red blood cells. Eyes are fully formed. The fetus is 9
inches long and weighs 21.6 oz.
Viability – The ability of the fetus to survive outside the womb; at
about week 24 to 25.
By 40 weeks: The fetus gains 50% of its birth weight in the last
month. The skin is smooth. The head has hair. Myelination
increases. The fetus is the size of the average newborn: roughly
17 – 20 inches long and weighing about 7 lbs.

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Periods of Prenatal Development
Brain Growth in the
Prenatal Period